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Bacteria (; common noun bacteria, singular bacterium) are a type of
biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to as the "building blocks of life". The s ...
. They constitute a large
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function *Doma ...
of
prokaryotic A prokaryote is a typically unicellular organism that lacks a nuclear membrane-enclosed nucleus. The word ''prokaryote'' comes from the Greek (, 'before') and (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connections". Pearson Education ...
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in older t ...
s. Typically a few
micrometre The micrometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is an SI derived unit of length equalling (SI standard pre ...
s in length, bacteria have a number of shapes, ranging from
spheres of a sphere A sphere (from Greek language, Greek —, "globe, ball") is a geometrical object in three-dimensional space that is the surface of a ball (viz., analogous to the circular objects in two dimensions, where a "circle" circumscribes its " ...
to rods and
spirals In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continents and islands. The remaining 71% is covered with water, mostly by oceans, seas, gulfs, an ...
, and are present in most of its
habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
s. Bacteria inhabit soil, water,
acidic hot springs
acidic hot springs
,
radioactive waste Radioactive waste is a type of hazardous waste that contains radioactive material. Radioactive waste is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, rare-earth mining, and nuclear weapons repr ...
, and the
deep biosphere The deep biosphere is the part of the biosphere that resides below the first few meters of the surface. It extends down at least 5 kilometers below the continental surface and 10.5 kilometers below the sea surface, at temperatures that m ...
of the
earth's crust 350px, Plates in the crust of Earth Earth's crust is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper ...
. Bacteria also live in
symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic. ...
and
parasitic Parasitism is a symbiotic relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the
bacterial phyla Bacterial phyla constitute the major lineages of the domain ''Bacteria''. While the exact definition of a bacterial phylum is debated, a popular definition is that a bacterial phylum is a monophyletic lineage of bacteria whose 16S rRNA genes shar ...
have species that can be grown in the laboratory. The study of bacteria is known as
bacteriology Bacteriology is the branch and specialty of biology that studies the morphology, ecology, genetics and biochemistry of bacteria as well as many other aspects related to them. This subdivision of microbiology involves the identification, classifica ...
, a branch of
microbiology Microbiology (from Greek , ''mīkros'', "small"; , ''bios'', "life"; and , ''-logia'') is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompa ...
. Nearly all animal life is dependent on bacteria for survival as only bacteria and some
archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
possess the genes and enzymes necessary to synthesize
vitamin B12
vitamin B<sub>12</sub>
, also known as
cobalamin Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in the metabolism of every cell of the human body. It is one of eight B vitamins. It is a cofactor in DNA synthesis, and in both fatty acid and amino acid metabolism. It ...

cobalamin
, and provide it through the food chain. Vitamin B12 is a water-soluble
vitamin A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an essential micronutrient which an organism needs in small quantities for the proper functioning of its metabolism. Essential nutrients ...
that is involved in the
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; the conve ...

metabolism
of every
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
of the human body. It is a cofactor in
DNA synthesis DNA synthesis is the natural or artificial creation of deoxyribonucleic acid (DNA) molecules. DNA is a macromolecule made up of nucleotide units, which are linked by covalent bonds and hydrogen bonds, in a repeating structure. DNA synthesis occurs w ...
, and in both
fatty acid fatty acids have perfectly straight chain structure. Unsaturated ones are typically bent, unless they have a trans configuration. In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain, which is ...
and
amino acid metabolismProtein metabolism denotes the various biochemical processes responsible for the synthesis of proteins and amino acids (anabolism), and the breakdown of proteins by catabolism. The steps of protein synthesis include transcription, translation, and p ...
. It is particularly important in the normal functioning of the
nervous system In biology, the nervous system is a highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that im ...
via its role in the synthesis of myelin. There are typically 40 million bacterial
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
in a gram of soil and a million bacterial cells in a millilitre of
fresh water Fresh water (or freshwater) is any naturally occurring water except seawater and brackish water. Fresh water is generally characterized by having low concentrations of dissolved salts and other total dissolved solids. Though the term specifical ...
. There are approximately 5×1030 bacteria on Earth, forming a
biomass Biomass is plant or animal material used as fuel to produce electricity or heat. Examples are wood, energy crops and waste from forests, yards, or farms. Since biomass technically can be used as a fuel directly (e.g. wood logs), some people use th ...
which is only exceeded by plants. Bacteria are vital in many stages of the
nutrient cycle A nutrient cycle (or ecological recycling) is the movement and exchange of organic and inorganic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyclic. ...

nutrient cycle
by recycling nutrients such as the fixation of nitrogen from the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in pla ...

atmosphere
. The nutrient cycle includes the
decomposition Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is essential f ...
of
dead bodies A cadaver or corpse is a dead human body that is used by medical students, physicians and other scientists to study anatomy, identify disease sites, determine causes of death, and provide tissue to repair a defect in a living human being. Student ...
; bacteria are responsible for the
putrefaction Putrefaction is the fifth stage of death, following pallor mortis, algor mortis, rigor mortis, and livor mortis. This process references the breaking down of a body of an animal such as a human post-mortem (meaning after death). In broad terms, ...
stage in this process. In the biological communities surrounding
hydrothermal vent A hydrothermal vent is a fissure on the seafloor from which geothermally heated water discharges. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart at spreading centers, ocean basin ...
s and
cold seep A cold seep (sometimes called a cold vent) is an area of the ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage occurs, often in the form of a brine pool. ''Cold'' does not mean that the temperature of the seepa ...
s,
extremophile An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism with optimal growth in environmental conditions considered extreme in that it is challenging for a carbon-based life form, such as all known life on Ear ...
bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as
hydrogen sulphide Hydrogen sulfide is the chemical compound with the formula . It is a colorless chalcogen hydride gas with the characteristic foul odor of rotten eggs. It is poisonous, corrosive, and flammable. Hydrogen sulfide is often produced from the microbia ...
and
methane Methane ( or ) is a chemical compound with the chemical formula (one atom of carbon and four atoms of hydrogen). It is a group-14 hydride and the simplest alkane, and is the main constituent of natural gas. The relative abundance of methane on ...
, to energy. In humans and most animals, the largest number of bacteria exist in the gut, and a large number on the
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different develo ...

skin
. The vast majority of the bacteria in the body are rendered harmless by the protective effects of the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinters, d ...
, though many are beneficial, particularly in the gut flora. However, several species of bacteria are
pathogenic In biology, a pathogen ( el, πάθος ''pathos'' "suffering", "passion" and -γενής ''-genēs'' "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious age ...
and cause
infectious disease An infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. An infectious disease, also known as a transmissib ...
s, including
cholera Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and mu ...

cholera
,
syphilis Syphilis is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, and tert ...
,
anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day to over two months after the infection is contracted. The skin form ...

anthrax
,
leprosy Leprosy, also known as Hansen's disease (HD), is a long-term infection by the bacteria ''Mycobacterium leprae'' or ''Mycobacterium lepromatosis''. Infection can lead to damage of the nerves, respiratory tract, skin, and eyes. This nerve damage ma ...

leprosy
, and
bubonic plague Bubonic plague is one of three types of plague caused by the plague bacterium (''Yersinia pestis''). One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and vomiting. Swollen an ...
. The most common fatal bacterial diseases are
respiratory infection Respiratory tract infections (RTIs) are infectious diseases involving the respiratory tract. An infection of this type usually is further classified as an upper respiratory tract infection (URI or URTI) or a lower respiratory tract infection (LRI o ...
s.
Tuberculosis Tuberculosis (TB) is an infectious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but can also affect other parts of the body. Most infections show no symptoms, in which cas ...
alone kills about 2 million people per year, mostly in
sub-Saharan Africa Sub-Saharan Africa is, geographically and ethnoculturally, the area of the continent of Africa that lies south of the Sahara. According to the United Nations, it consists of all African countries and territories that are fully or partially so ...
.
Antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of su ...
s are used to treat
bacterial infections Pathogenic bacteria are bacteria that can cause disease. This article deals with human pathogenic bacteria. Although most bacteria are harmless or often beneficial, some are pathogenic, with the number of species estimated as fewer than a hundre ...
and are also used in farming, making
antibiotic resistance Antimicrobial resistance (AMR or AR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. The term antibiotic resistance (AR or ABR) is a subset of AMR, as it applies to bacteria that become resistant to ant ...

antibiotic resistance
a growing problem. In industry, bacteria are important in
sewage treatment#REDIRECT Sewage treatment#REDIRECT Sewage treatment {{R from other capitalisation ...
{{R from other capitalisation ...
and the breakdown of
oil spill An oil spill is the release of a liquid petroleum hydrocarbon into the environment, especially the marine ecosystem, due to human activity, and is a form of pollution. The term is usually given to marine oil spills, where oil is released into the ...
s, the production of
cheese Cheese is a dairy product, derived from milk and produced in wide ranges of flavors, textures and forms by coagulation of the milk protein casein. It comprises proteins and fat from milk, usually the milk of cows, buffalo, goats, or sheep. Du ...

cheese
and
yogurt Yogurt (; , from tr, yoğurt, arm, յոգուրտ) also spelled yoghurt, yogourt or yoghourt, is a food produced by bacterial fermentation of milk. The bacteria used to make yogurt are known as ''yogurt cultures''. Fermentation of sugars in th ...
through
fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food prod ...
, the recovery of gold, palladium, copper and other metals in the mining sector, as well as in
biotechnology Biotechnology is a broad area of biology, involving the use of living systems and organisms to develop or make products. Depending on the tools and applications, it often overlaps with related scientific fields. In the late 20th and early 21st c ...
, and the manufacture of antibiotics and other chemicals. Once regarded as
plant Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi wer ...
s constituting the class ''Schizomycetes'' ("fission fungi"), bacteria are now classified as
prokaryotes A prokaryote is a typically unicellular organism that lacks a nuclear membrane-enclosed nucleus. The word ''prokaryote'' comes from the Greek (, 'before') and (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connections". Pearson Education ...
. Unlike cells of animals and other
eukaryote Eukaryotes () are organisms whose cells have a nucleus enclosed within a nuclear envelope. Eukaryotes belong to the domain Eukaryota or Eukarya; their name comes from the Greek εὖ (''eu'', "well" or "good") and κάρυον (''karyon'', "nu ...
s, bacterial cells do not contain a
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
and rarely harbour
membrane-bound A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates cell from the external environment or creates intracellular compartments. Biological membranes, in the form of eukaryotic cell membranes, consis ...
organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the su ...
s. Although the term ''bacteria'' traditionally included all prokaryotes, the
scientific classification In biology, taxonomy () is the scientific study of naming, defining (circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...
changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that
evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Different character ...

evolved
from an ancient common ancestor. These evolutionary domains are called ''Bacteria'' and ''
Archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
''.


Etymology

The word ''bacteria'' is the plural of the
New Latin New Latin (also called Neo-Latin or Modern Latin) is the revival of Latin used in original, scholarly, and scientific works since about 1500. Modern scholarly and technical nomenclature, such as in zoological and botanical taxonomy and internati ...
', which is the latinisation of the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of ...
(''bakterion''), the diminutive of βακτηρία (''bakteria''), meaning "staff, cane", because the first ones to be discovered were
rod-shaped A bacillus (plural bacilli), or bacilliform bacterium, is a rod-shaped bacterium or archaeon. Bacilli are found in many different taxonomic groups of bacteria. However, the name ''Bacillus'', capitalized and italicized, refers to a specific genus ...
.


Origin and early evolution

The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life. Although bacterial
fossil A fossil (from Classical Latin: , literally "obtained by digging") is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or ...
s exist, such as
stromatolite Stromatolites () or stromatoliths (from Greek "layer, stratum" (GEN ), and "rock") are layered sedimentary formations that are created by photosynthetic cyanobacteria. These microorganisms produce adhesive compounds that cement sand and oth ...
s, their lack of distinctive
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial
phylogeny , based on completely sequenced genomes. A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary rela ...

phylogeny
, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage. The
most recent common ancestor In biology and genealogy, the most recent common ancestor (MRCA), ''last common ancestor'' (LCA), or ''concestor'' of a set of organisms is the most recent individual from which all the organisms of the set are descended. The term is also used in ...
of bacteria and archaea was probably a
hyperthermophile A hyperthermophile is an organism that thrives in extremely hot environments—from 60 °C (140 °F) upwards. An optimal temperature for the existence of hyperthermophiles is often above 80 °C (176 °F). Hyperthermophiles are often within the domai ...
that lived about 2.5 billion–3.2 billion years ago. The earliest life on land may have been bacteria some 3.22 billion years ago. Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into
endosymbiotic An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within", σύν ' ...
associations with the ancestors of eukaryotic cells, which were themselves possibly related to the
Archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
. This involved the engulfment by proto-eukaryotic cells of
alphaproteobacteria Alphaproteobacteria is a class of bacteria in the phylum Proteobacteria (See also bacterial taxonomy). Its members are highly diverse and possess few commonalities, but nevertheless share a common ancestor. Like all ''Proteobacteria'', its members ...
l
symbionts Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic. ...
to form either
mitochondria A mitochondrion (, plural mitochondria) is a double membrane-bound organelle found in most eukaryotic organisms. Some cells in some multicellular organisms lack mitochondria (for example, mature mammalian red blood cells). A number of unicellula ...
or
hydrogenosome A hydrogenosome is a membrane-enclosed organelle found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from mitochondria to produce molecular hydrogen and ATP in anaerob ...
s, which are still found in all known Eukarya (sometimes in highly
reduced formIn statistics, and particularly in econometrics, the reduced form of a system of equations is the result of solving the system for the endogenous variables. This gives the latter as functions of the exogenous variables, if any. In econometrics, the ...
, e.g. in ancient "amitochondrial" protozoa). Later, some eukaryotes that already contained mitochondria also engulfed
cyanobacteria Cyanobacteria , also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' comes from their color ( el, κυανός, translit=kyanós, translation=blue), giving them their ot ...
-like organisms, leading to the formation of
chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water ...

chloroplast
s in algae and plants. This is known as primary endosymbiosis.


Morphology

Bacteria display a wide diversity of shapes and sizes, called morphologies. Bacterial cells are about one-tenth the size of eukaryotic cells and are typically 0.5–5.0 
micrometre The micrometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is an SI derived unit of length equalling (SI standard pre ...
s in length. However, a few species are visible to the unaided eye—for example, ''
Thiomargarita namibiensis ''Thiomargarita namibiensis'' is a Gram-negative coccoid Proteobacterium, found in the ocean sediments of the continental shelf of Namibia. It is the largest bacterium ever discovered, as a rule in diameter, but sometimes attaining . Cells of '' ...
'' is up to half a millimetre long and ''
Epulopiscium fishelsoni ''Epulopiscium spp.'' are a group of Gram-positive bacteria that have a symbiotic relationship with surgeonfish. These bacteria are known for their unusually large size, many ranging from 200–700 μm in length. Until the discovery of ''Thiomarga ...
'' reaches 0.7 mm. Among the smallest bacteria are members of the genus ''
Mycoplasma ''Mycoplasma'' (plural mycoplasmas or mycoplasmata) is a genus of bacteria that lack a cell wall around their cell membranes. This characteristic makes them naturally resistant to antibiotics that target cell wall synthesis (like the beta-lact ...
'', which measure only 0.3 micrometres, as small as the largest
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovs ...
es. Some bacteria may be even smaller, but these ultramicrobacteria are not well-studied. Most bacterial species are either spherical, called ''
cocci A coccus (plural cocci) is any bacterium or archaeon that has a spherical, ovoid, or generally round shape. Bacteria are categorized based on their shapes into three classes: cocci (spherical-shaped), bacillus (rod-shaped) and spirochetes (spir ...
'' (''singular coccus'', from Greek ''kókkos'', grain, seed), or rod-shaped, called ''
bacilli Bacilli is a taxonomic class of bacteria that includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens such as ''Bacillus anthracis'' (the cause of anthrax). ''Bacilli'' are almost exclusively gram-positive ...
'' (''sing''. bacillus, from
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the dominant language ...
''baculus'', stick). Some bacteria, called ''
vibrio ''Vibrio'' is a genus of Gram-negative bacteria, possessing a curved-rod (comma) shape, several species of which can cause foodborne infection, usually associated with eating undercooked seafood. Typically found in salt water, ''Vibrio'' species ...
'', are shaped like slightly curved rods or comma-shaped; others can be spiral-shaped, called '' spirilla'', or tightly coiled, called ''
spirochaetes A spirochaete () or spirochete is a member of the phylum Spirochaetes (), which contains distinctive diderm (double-membrane) bacteria, most of which have long, helically coiled (corkscrew-shaped or spiraled, hence the name) cells. Spirochaetes ...
''. A small number of other unusual shapes have been described, such as star-shaped bacteria. This wide variety of shapes is determined by the bacterial
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mecha ...
and
cytoskeleton 300px, The eukaryotic cytoskeleton. Actin filaments are shown in red, and microtubules composed of beta tubulin are in green. The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all ...
, and is important because it can influence the ability of bacteria to acquire nutrients, attach to surfaces, swim through liquids and escape
predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the hos ...
. Many bacterial species exist simply as single cells, others associate in characteristic patterns: ''
Neisseria ''Neisseria'' is a large genus of bacteria that colonize the mucosal surfaces of many animals. Of the 11 species that colonize humans, only two are pathogens, ''N. meningitidis'' and ''N. gonorrhoeae''. Most gonococcal infections are asymptomati ...
'' form diploids (pairs), ''
Streptococcus ''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Firmicutes. Cell division in streptococci occurs ...
'' form chains, and ''
Staphylococcus ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...
'' group together in "bunch of grapes" clusters. Bacteria can also group to form larger multicellular structures, such as the elongated filaments of ''
Actinobacteria The Actinobacteria are a phylum of mostly Gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to soil systems. In soil they help ...
'', the aggregates of ''
Myxobacteria The myxobacteria ("slime bacteria") are a group of bacteria that predominantly live in the soil and feed on insoluble organic substances. The myxobacteria have very large genomes relative to other bacteria, e.g. 9–10 million nucleotides except ...
'', and the complex hyphae of ''
Streptomyces ''Streptomyces'' is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, a ...
''. These multicellular structures are often only seen in certain conditions. For example, when starved of amino acids,
Myxobacteria The myxobacteria ("slime bacteria") are a group of bacteria that predominantly live in the soil and feed on insoluble organic substances. The myxobacteria have very large genomes relative to other bacteria, e.g. 9–10 million nucleotides except ...
detect surrounding cells in a process known as
quorum sensingIn biology, quorum sensing (or quorum signalling) is the ability to detect and respond to cell population density by gene regulation. As one example, quorum sensing (QS) enables bacteria to restrict the expression of specific genes to the high cell d ...
, migrate towards each other, and aggregate to form fruiting bodies up to 500 micrometres long and containing approximately 100,000 bacterial cells. In these fruiting bodies, the bacteria perform separate tasks; for example, about one in ten cells migrate to the top of a fruiting body and differentiate into a specialised dormant state called a myxospore, which is more resistant to drying and other adverse environmental conditions. Bacteria often attach to surfaces and form dense aggregations called
biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular polymeric s ...

biofilm
s, and larger formations known as
microbial mat The cyanobacterial algal mat, salty lake on the White Sea">algal_mat.html" style="text-decoration: none;"class="mw-redirect" title="cyanobacterial algal mat">cyanobacterial algal mat, salty lake on the White Sea seaside A microbial mat is ...
s. These biofilms and mats can range from a few micrometres in thickness to up to half a metre in depth, and may contain multiple species of bacteria,
protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exclus ...
s and
archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
. Bacteria living in biofilms display a complex arrangement of cells and extracellular components, forming secondary structures, such as microcolonies, through which there are networks of channels to enable better diffusion of nutrients. In natural environments, such as soil or the surfaces of plants, the majority of bacteria are bound to surfaces in biofilms. Biofilms are also important in medicine, as these structures are often present during chronic bacterial infections or in infections of implanted
medical device Artificial pacemaker, a Class III device in the United States A medical device is any device intended to be used for medical purposes. Medical devices benefit patients by helping health care providers diagnose and treat patients and helping patie ...
s, and bacteria protected within biofilms are much harder to kill than individual isolated bacteria.


Cellular structure


Intracellular structures

The bacterial cell is surrounded by a
cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates the interior of all cells from the outside ...
, which is made primarily of
phospholipid Phospholipids, also known as phosphatides, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group, and two hydrophobic "tails" derived from fatty acids, joined by a glycerol molecule. The phosphate group can be ...

phospholipid
s. This membrane encloses the contents of the cell and acts as a barrier to hold nutrients,
protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s and other essential components of the
cytoplasm In cell biology, the cytoplasm is all of the material within a cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The main compone ...
within the cell. Unlike
eukaryotic cells Eukaryotes () are organisms whose cells have a nucleus enclosed within a nuclear envelope. Eukaryotes belong to the domain Eukaryota or Eukarya; their name comes from the Greek εὖ (''eu'', "well" or "good") and κάρυον (''karyon'', "nu ...
, bacteria usually lack large membrane-bound structures in their cytoplasm such as a
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
,
mitochondria A mitochondrion (, plural mitochondria) is a double membrane-bound organelle found in most eukaryotic organisms. Some cells in some multicellular organisms lack mitochondria (for example, mature mammalian red blood cells). A number of unicellula ...
,
chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water ...

chloroplast
s and the other organelles present in eukaryotic cells. However, some bacteria have protein-bound organelles in the cytoplasm which compartmentalize aspects of bacterial metabolism, such as the
carboxysome 460px, Electron micrographs showing alpha-carboxysomes from the chemoautotrophic bacterium ''Halothiobacillus neapolitanus'': (A) arranged within the cell, and (B) intact upon isolation. Scale bars indicate 100 nm. Carboxysomes are bacterial microco ...
. Additionally, bacteria have a multi-component
cytoskeleton 300px, The eukaryotic cytoskeleton. Actin filaments are shown in red, and microtubules composed of beta tubulin are in green. The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all ...
to control the localisation of proteins and nucleic acids within the cell, and to manage the process of
cell division Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative division, whereby ...
. Many important
biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
reactions, such as energy generation, occur due to
concentration gradients
concentration gradients
across membranes, creating a
potential Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple rel ...
difference analogous to a battery. The general lack of internal membranes in bacteria means these reactions, such as
electron transport is the site of oxidative phosphorylation in eukaryotes. The NADH and succinate generated in the citric acid cycle are oxidized, providing energy to power ATP synthase. Image:Thylakoid membrane 3.svg">400px, Photosynthetic electron transport chain of ...

electron transport
, occur across the cell membrane between the cytoplasm and the outside of the cell or
periplasm400px, cell_wall.html"_style="text-decoration:_none;"class="mw-redirect"_title="Gram-negative_cell_wall">Gram-negative_cell_wall_ The_periplasm_is_a_concentrated_gel-like_matrix_(biology).html" style="text-decoration: none;"class="mw-redirect" ...
. However, in many photosynthetic bacteria the plasma membrane is highly folded and fills most of the cell with layers of light-gathering membrane. These light-gathering complexes may even form lipid-enclosed structures called
chlorosome A chlorosome is a photosynthetic antenna complex found in green sulfur bacteria (GSB) and some green filamentous anoxygenic phototrophs (FAP) (Chloroflexaceae, Oscillochloridaceae; both members of Chloroflexia). They differ from other antenna comple ...
s in
green sulfur bacteria The green sulfur bacteria (Chlorobiaceae) are a family of obligately anaerobic photoautotrophic bacteria. Together with the non-photosynthetic Ignavibacteriaceae, they form the phylum Chlorobi. Green sulfur bacteria are nonmotile (except ''Chloro ...
. Bacteria do not have a membrane-bound nucleus, and their
gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." (Greek) meaning ''generation'' or ''birth'' ) is a basic unit of heredity and a sequence of nucleotides in DNA or R ...
tic material is typically a single
circular bacterial chromosome A circular chromosome is a chromosome in bacteria, archaea, mitochondria, and chloroplasts, in the form of a molecule of circular DNA, unlike the linear chromosome of most eukaryotes. Most prokaryote chromosomes contain a circular DNA molecule – ...
of
DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, g ...
located in the cytoplasm in an irregularly shaped body called the
nucleoid The nucleoid (meaning ''nucleus-like'') is an irregularly shaped region within the prokaryotic cell that contains all or most of the genetic material. The chromosome of a prokaryote is circular, and its length is very large compared to the cell dime ...
. The nucleoid contains the
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by chaperone proteins, bind to and condense the DNA molecule to m ...

chromosome
with its associated proteins and
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid (DNA) are nucleic acids. Along with lipids, proteins, and carbohydrates ...

RNA
. Like all other
organism In biology, an organism (from Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the properties of life. It is a synonym for "life form". Organisms are classified by taxonomy into groups such as ...
s, bacteria contain
ribosome Ribosomes () are macromolecular machines, found within all living cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to f ...
s for the production of proteins, but the structure of the bacterial ribosome is different from that of
eukaryote Eukaryotes () are organisms whose cells have a nucleus enclosed within a nuclear envelope. Eukaryotes belong to the domain Eukaryota or Eukarya; their name comes from the Greek εὖ (''eu'', "well" or "good") and κάρυον (''karyon'', "nu ...
s and
Archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
. Some bacteria produce intracellular nutrient storage granules, such as
glycogen A view of the atomic structure of a single branched strand of glucose units in a glycogen molecule. Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The p ...

glycogen
,
polyphosphatePolyphosphates are salts or esters of polymeric oxyanions formed from tetrahedral PO4 (phosphate) structural units linked together by sharing oxygen atoms. Polyphosphates can adopt linear or a cyclic ring structures. In biology, the polyphosphate est ...
,
sulfur Sulfur (in traditional lay Commonwealth English: sulphur) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecul ...
or
polyhydroxyalkanoates ), a polyhydroxyalkanoate Polyhydroxyalkanoates or PHAs are polyesters produced in nature by numerous microorganisms, including through bacterial fermentation of sugars or lipids. When produced by bacteria they serve as both a source of energy and ...
. Bacteria such as the
photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's metabolic activities. This chemical energy is stored in ...

photosynthetic
cyanobacteria Cyanobacteria , also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' comes from their color ( el, κυανός, translit=kyanós, translation=blue), giving them their ot ...
, produce internal gas vacuoles, which they use to regulate their buoyancy, allowing them to move up or down into water layers with different light intensities and nutrient levels.


Extracellular structures

Around the outside of the cell membrane is the
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mecha ...
. Bacterial cell walls are made of
peptidoglycan Peptidoglycan or murein is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria, forming the cell wall. The sugar component consists of alternating residues of β-(1,4) linked ''N' ...
(also called murein), which is made from
polysaccharide , a beta-glucan polysaccharide Image:amylose 3Dprojection.svg">350px, Amylose is a linear polymer of glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of starch, th ...
chains cross-linked by
peptide Peptides (from Greek language πεπτός, ''peptós'' "digested"; derived from πέσσειν, ''péssein'' "to digest") are short chains of between two and fifty amino acids, linked by peptide bonds. Chains of fewer than ten or fifteen amino ac ...
s containing D-
amino acid Amino acids are organic compounds that contain amino (–NH2) and carboxyl (–COOH) functional groups, along with a side chain (R group) specific to each amino acid. The key elements of an amino acid are carbon (C), hydrogen (H), oxygen (O), a ...
s. Bacterial cell walls are different from the cell walls of
plant Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi wer ...
s and
fungi A fungus (plural: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, which is separa ...
, which are made of
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of ...
and
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin (carbon, C8H13O5N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, a derivative of glucose. This polysaccharide is a primary component of cel ...

chitin
, respectively. The cell wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, and the antibiotic
penicillin Penicillins (P, PCN or PEN) are a group of antibiotics originally obtained from ''Penicillium'' moulds, principally ''P. chrysogenum'' and ''P. rubens''. Most penicillins in clinical use are chemically synthesised from naturally-produced pen ...

penicillin
(produced by a fungus called ''Penicillium'') is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan. There are broadly speaking two different types of cell wall in bacteria, that classify bacteria into
Gram-positive bacteria 300px, Violet-stained gram-positive bacilli_.html" style="text-decoration: none;"class="mw-redirect" title="gram-negative">cocci and pink-stained bacilli_">gram-negative">cocci_and_pink-stained_gram-negative_bacillus_(shape)">bacilli_ In_bacte ...
and
Gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the gram-staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall san ...
. The names originate from the reaction of cells to the
Gram stain Gram stain or Gram staining, also called Gram's method, is a method of staining used to distinguish and classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. The name comes from the Danish bacteriolo ...
, a long-standing test for the classification of bacterial species. Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and
teichoic acid Teichoic acids (''cf.'' Greek τεῖχος, ''teīkhos'', "wall", to be specific a fortification wall, as opposed to τοῖχος, ''toīkhos'', a regular wall) are bacterial copolymers of glycerol phosphate or ribitol phosphate and carbohydrat ...
s. In contrast, Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second
lipid membrane The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many virus ...
containing
lipopolysaccharide Lipopolysaccharides (LPS), also known as endotoxins, are large molecules consisting of a lipid and a polysaccharide composed of O-antigen, outer core and inner core joined by a covalent bond; they are found in the outer membrane of Gram-negative ...
s and
lipoprotein 250px, Structure of a chylomicron. ApoA, ApoB, ApoC, ApoE are apolipoproteins; green particles are phospholipids; T is triacylglycerol; C is cholesterol ester. A lipoprotein is a biochemical assembly whose primary function is to transport ...
s. Most bacteria have the Gram-negative cell wall, and only the
Firmicutes The Firmicutes (Latin: ''firmus'', strong, and ''cutis'', skin, referring to the cell wall) are a phylum of bacteria, most of which have gram-positive cell wall structure. A few, however, such as ''Megasphaera'', ''Pectinatus'', ''Selenomonas'' and ...
and
Actinobacteria The Actinobacteria are a phylum of mostly Gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to soil systems. In soil they help ...
(previously known as the low G+C and high G+C Gram-positive bacteria, respectively) have the alternative Gram-positive arrangement. These differences in structure can produce differences in antibiotic susceptibility; for instance,
vancomycin Vancomycin is an antibiotic medication used to treat a number of bacterial infections. It is recommended intravenously as a treatment for complicated skin infections, bloodstream infections, endocarditis, bone and joint infections, and meningitis ...

vancomycin
can kill only Gram-positive bacteria and is ineffective against Gram-negative
pathogen In biology, a pathogen ( el, πάθος ''pathos'' "suffering", "passion" and -γενής ''-genēs'' "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious age ...
s, such as ''
Haemophilus influenzae#REDIRECT Haemophilus influenzae#REDIRECT Haemophilus influenzae {{R from other capitalisation ...
{{R from other capitalisation ...
'' or ''
Pseudomonas aeruginosa ''Pseudomonas aeruginosa'' is a common encapsulated, Gram-negative, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. aeruginosa'' is a multidrug resistant path ...

Pseudomonas aeruginosa
''. Some bacteria have cell wall structures that are neither classically Gram-positive or Gram-negative. This includes clinically important bacteria such as ''
Mycobacteria ''Mycobacterium'' is a genus of Actinobacteria, given its own family, the Mycobacteriaceae. Over 190 species are recognized in this genus. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis (''Mycobact ...
'' which have a thick peptidoglycan cell wall like a Gram-positive bacterium, but also a second outer layer of lipids. In many bacteria, an
S-layerAn S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria. It consists of a monomolecular layer composed of identical proteins or glycoproteins. This structure is built via self-assemb ...
of rigidly arrayed protein molecules covers the outside of the cell. This layer provides chemical and physical protection for the cell surface and can act as a
macromolecular macromolecule A macromolecule is a very large molecule, such as a protein. They are composed of thousands of covalently bonded atoms. Many macromolecules are the polymerization of smaller molecules called monomers. The most common macromolecule ...
diffusion barrierA diffusion barrier is a thin layer (usually micrometres thick) of metal usually placed between two other metals. It is done to act as a barrier to protect either one of the metals from corrupting the other.. Adhesion of a plated metal layer to its ...
. S-layers have diverse but mostly poorly understood functions, but are known to act as virulence factors in ''
Campylobacter ''Campylobacter'' (meaning "curved bacteria") is a genus of Gram-negative bacteria. ''Campylobacter'' typically appear comma- or s-shaped, and are motile. Some ''Campylobacter'' species can infect humans, sometimes causing campylobacteriosis, a di ...
'' and contain surface
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules k ...
s in ''
Bacillus stearothermophilus ''Geobacillus stearothermophilus'' (previously ''Bacillus stearothermophilus'') is a rod-shaped, Gram-positive bacterium and a member of the division Firmicutes. The bacterium is a thermophile and is widely distributed in soil, hot springs, ocean ...
''.
Flagella A flagellum (; plural: flagella) is a lash-like appendage that protrudes from the cell body of certain cells termed as flagellates. A flagellate can have one or several flagella. The primary function of a flagellum is that of locomotion, but it ...
are rigid protein structures, about 20 nanometres in diameter and up to 20 micrometres in length, that are used for
motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms that ...
. Flagella are driven by the energy released by the transfer of
ion An ion () is a particle, atom or molecule with a net electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is equal and opposite to charged proton(s) considered positive by convent ...
s down an
electrochemical gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and the ...
across the cell membrane. Fimbriae (sometimes called " attachment pili") are fine filaments of protein, usually 2–10 nanometres in diameter and up to several micrometres in length. They are distributed over the surface of the cell, and resemble fine hairs when seen under the
electron microscope An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a high ...

electron microscope
. Fimbriae are believed to be involved in attachment to solid surfaces or to other cells, and are essential for the virulence of some bacterial pathogens. Pili (''sing''. pilus) are cellular appendages, slightly larger than fimbriae, that can transfer
genetic material#REDIRECT Nucleic acid#REDIRECT Nucleic acid#REDIRECT Nucleic acid {{Redirect category shell, 1= {{R from other capitalisation ... {{Redirect category shell, 1= {{R from other capitalisation ...
{{Redirect category shell, 1= {{R from other ca ...
between bacterial cells in a process called
conjugation Conjugation or conjugate may refer to: Linguistics * Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics * Complex conjugation, the change ...
where they are called conjugation pili or sex pili (see bacterial genetics, below). They can also generate movement where they are called
type IV pili Image:Conjugation.svg, 350px, Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferr ...
.
Glycocalyx The glycocalyx, also known as the pericellular matrix, is a glycoprotein and glycolipid covering that surrounds the cell membranes of some bacteria, epithelia, and other cells. In 1970, Martinez-Palomo discovered the cell coating in animal cells, w ...
is produced by many bacteria to surround their cells, and varies in structural complexity: ranging from a disorganised slime layer of extracellular polymeric substances to a highly structured bacterial capsule, capsule. These structures can protect cells from engulfment by eukaryotic cells such as macrophages (part of the human
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinters, d ...
). They can also act as antigens and be involved in cell recognition, as well as aiding attachment to surfaces and the formation of biofilms. The assembly of these extracellular structures is dependent on bacterial secretion systems. These transfer proteins from the cytoplasm into the periplasm or into the environment around the cell. Many types of secretion systems are known and these structures are often essential for the virulence of pathogens, so are intensively studied.


Endospores

Certain Genus, genera of Gram-positive bacteria, such as ''Bacillus'', ''Clostridium'', ''Sporohalobacter'', ''Anaerobacter'', and ''Heliobacteria, Heliobacterium'', can form highly resistant, dormant structures called ''endospores''. Endospores develop within the cytoplasm of the cell; generally a single endospore develops in each cell. Each endospore contains a core of
DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, g ...
and
ribosome Ribosomes () are macromolecular machines, found within all living cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to f ...
s surrounded by a cortex layer and protected by a multilayer rigid coat composed of peptidoglycan and a variety of proteins. Endospores show no detectable
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; the conve ...

metabolism
and can survive extreme physical and chemical stresses, such as high levels of ultraviolet, UV light, gamma ray, gamma radiation, detergents, disinfectants, heat, freezing, pressure, and desiccation. In this dormant state, these organisms may remain viable for millions of years, and endospores even allow bacteria to survive exposure to the Hard vacuum, vacuum and radiation in space, possibly bacteria could be distributed throughout the Universe by space dust, meteoroids, asteroids, comets, Small Solar System body, planetoids or via directed panspermia. Endospore-forming bacteria can also cause disease: for example,
anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day to over two months after the infection is contracted. The skin form ...

anthrax
can be contracted by the inhalation of ''Bacillus anthracis'' endospores, and contamination of deep puncture wounds with ''Clostridium tetani'' endospores causes tetanus.


Metabolism

Bacteria exhibit an extremely wide variety of metabolism, metabolic types. The distribution of metabolic traits within a group of bacteria has traditionally been used to define their Taxonomy (biology), taxonomy, but these traits often do not correspond with modern genetic classifications. Bacterial metabolism is classified into primary nutritional groups, nutritional groups on the basis of three major criteria: the source of Energy (biology), energy, the electron donors used, and the source of carbon used for growth. Bacteria either derive energy from light using photosynthesis (called phototrophy), or by breaking down chemical compounds using oxidation (called chemotrophy). Chemotrophs use chemical compounds as a source of energy by transferring electrons from a given electron donor to a terminal electron acceptor in a redox, redox reaction. This reaction releases energy that can be used to drive metabolism. Chemotrophs are further divided by the types of compounds they use to transfer electrons. Bacteria that use inorganic compounds such as hydrogen, carbon monoxide, or ammonia as electron donor, sources of electrons are called lithotrophs, while those that use organic compounds are called organotrophs. The compounds used to receive electrons are also used to classify bacteria: aerobic organisms use oxygen as the terminal electron acceptor, while anaerobic organisms use other compounds such as nitrate, sulfate, or carbon dioxide. Many bacteria get their carbon from other organic compound, organic carbon, called heterotrophy. Others such as
cyanobacteria Cyanobacteria , also known as Cyanophyta, are a phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' comes from their color ( el, κυανός, translit=kyanós, translation=blue), giving them their ot ...
and some purple bacteria are autotrophic, meaning that they obtain cellular carbon by Carbon fixation, fixing carbon dioxide. In unusual circumstances, the gas
methane Methane ( or ) is a chemical compound with the chemical formula (one atom of carbon and four atoms of hydrogen). It is a group-14 hydride and the simplest alkane, and is the main constituent of natural gas. The relative abundance of methane on ...
can be used by methanotrophic bacteria as both a source of electrons and a substrate for carbon anabolism. In many ways, bacterial metabolism provides traits that are useful for ecological stability and for human society. One example is that some bacteria have the ability to nitrogen fixation, fix nitrogen gas using the enzyme nitrogenase. This environmentally important trait can be found in bacteria of most metabolic types listed above. This leads to the ecologically important processes of denitrification, sulfate reduction, and acetogenesis, respectively. Bacterial metabolic processes are also important in biological responses to pollution; for example, sulfate-reducing bacteria are largely responsible for the production of the highly toxic forms of mercury (element), mercury (methylmercury, methyl- and dimethylmercury) in the environment. Non-respiratory anaerobes use
fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food prod ...
to generate energy and reducing power, secreting metabolic by-products (such as ethanol in brewing) as waste. Facultative anaerobes can switch between fermentation and different terminal electron acceptors depending on the environmental conditions in which they find themselves.


Growth and reproduction

Unlike in multicellular organisms, increases in cell size (cell growth) and reproduction by
cell division Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative division, whereby ...
are tightly linked in unicellular organisms. Bacteria grow to a fixed size and then reproduce through binary fission, a form of asexual reproduction. Under optimal conditions, bacteria can grow and divide extremely rapidly, and bacterial populations can double as quickly as every 9.8 minutes. In cell division, two identical clone (genetics), clone daughter cells are produced. Some bacteria, while still reproducing asexually, form more complex reproductive structures that help disperse the newly formed daughter cells. Examples include fruiting body formation by ''
Myxobacteria The myxobacteria ("slime bacteria") are a group of bacteria that predominantly live in the soil and feed on insoluble organic substances. The myxobacteria have very large genomes relative to other bacteria, e.g. 9–10 million nucleotides except ...
'' and aerial hyphae formation by ''
Streptomyces ''Streptomyces'' is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, a ...
'', or budding. Budding involves a cell forming a protrusion that breaks away and produces a daughter cell. In the laboratory, bacteria are usually grown using solid or liquid media. Solid Growth medium, growth media, such as agar plates, are used to Isolation (microbiology), isolate pure cultures of a bacterial strain. However, liquid growth media are used when the measurement of growth or large volumes of cells are required. Growth in stirred liquid media occurs as an even cell suspension, making the cultures easy to divide and transfer, although isolating single bacteria from liquid media is difficult. The use of selective media (media with specific nutrients added or deficient, or with antibiotics added) can help identify specific organisms. Most laboratory techniques for growing bacteria use high levels of nutrients to produce large amounts of cells cheaply and quickly. However, in natural environments, nutrients are limited, meaning that bacteria cannot continue to reproduce indefinitely. This nutrient limitation has led the evolution of different growth strategies (see r/K selection theory). Some organisms can grow extremely rapidly when nutrients become available, such as the formation of algal bloom, algal (and cyanobacterial) blooms that often occur in lakes during the summer. Other organisms have adaptations to harsh environments, such as the production of multiple antibiotics by ''
Streptomyces ''Streptomyces'' is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, a ...
'' that inhibit the growth of competing microorganisms. In nature, many organisms live in communities (e.g.,
biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular polymeric s ...

biofilm
s) that may allow for increased supply of nutrients and protection from environmental stresses. These relationships can be essential for growth of a particular organism or group of organisms (syntrophy). Bacterial growth follows four phases. When a population of bacteria first enter a high-nutrient environment that allows growth, the cells need to adapt to their new environment. The first phase of growth is the Stationary phase (biology), lag phase, a period of slow growth when the cells are adapting to the high-nutrient environment and preparing for fast growth. The lag phase has high biosynthesis rates, as proteins necessary for rapid growth are produced. The second phase of growth is the Stationary phase (biology), logarithmic phase, also known as the exponential phase. The log phase is marked by rapid exponential growth. The rate at which cells grow during this phase is known as the ''growth rate'' (''k''), and the time it takes the cells to double is known as the ''generation time'' (''g''). During log phase, nutrients are metabolised at maximum speed until one of the nutrients is depleted and starts limiting growth. The third phase of growth is the ''Stationary phase (biology), stationary phase'' and is caused by depleted nutrients. The cells reduce their metabolic activity and consume non-essential cellular proteins. The stationary phase is a transition from rapid growth to a stress response state and there is increased gene expression, expression of genes involved in DNA repair, antioxidant, antioxidant metabolism and active transport, nutrient transport. The final phase is the Stationary phase (biology), death phase where the bacteria run out of nutrients and die.


Genetics

Most bacteria have a single circular
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by chaperone proteins, bind to and condense the DNA molecule to m ...

chromosome
that can range in size from only 160,000 base pairs in the
endosymbiotic An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within", σύν ' ...
bacteria ''Candidatus Carsonella ruddii, Carsonella ruddii'', to 12,200,000 base pairs (12.2 Mbp) in the soil-dwelling bacteria ''Sorangium cellulosum''. There are many exceptions to this, for example some ''
Streptomyces ''Streptomyces'' is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, a ...
'' and ''Borrelia'' species contain a single linear chromosome, while some ''Vibrio'' species contain more than one chromosome. Bacteria can also contain plasmids, small extra-chromosomal molecules of DNA that may contain genes for various useful functions such as
antibiotic resistance Antimicrobial resistance (AMR or AR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. The term antibiotic resistance (AR or ABR) is a subset of AMR, as it applies to bacteria that become resistant to ant ...

antibiotic resistance
, metabolic capabilities, or various virulence, virulence factors. Bacteria genomes usually encode a few hundred to a few thousand genes. The genes in bacterial genomes are usually a single continuous stretch of DNA and although several different types of introns do exist in bacteria, these are much rarer than in eukaryotes. Bacteria, as asexual organisms, inherit an identical copy of the parent's genomes and are Clonal colony, clonal. However, all bacteria can evolve by selection on changes to their genetic material
DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, g ...
caused by genetic recombination or mutations. Mutations come from errors made during the replication of DNA or from exposure to mutagens. Mutation rates vary widely among different species of bacteria and even among different clones of a single species of bacteria. Genetic changes in bacterial genomes come from either random mutation during replication or "stress-directed mutation", where genes involved in a particular growth-limiting process have an increased mutation rate. Some bacteria also transfer genetic material between cells. This can occur in three main ways. First, bacteria can take up exogenous DNA from their environment, in a process called transformation (genetics), transformation. Many bacteria can natural competence, naturally take up DNA from the environment, while others must be chemically altered in order to induce them to take up DNA. The development of competence in nature is usually associated with stressful environmental conditions, and seems to be an adaptation for facilitating repair of DNA damage in recipient cells. The second way bacteria transfer genetic material is by transduction (genetics), transduction, when the integration of a bacteriophage introduces foreign DNA into the chromosome. Many types of bacteriophage exist, some simply infect and lytic cycle, lyse their host (biology), host bacteria, while others insert into the bacterial chromosome. Bacteria resist phage infection through restriction modification systems that degrade foreign DNA, and a system that uses CRISPR sequences to retain fragments of the genomes of phage that the bacteria have come into contact with in the past, which allows them to block virus replication through a form of RNA interference. The third method of gene transfer is
conjugation Conjugation or conjugate may refer to: Linguistics * Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics * Complex conjugation, the change ...
, whereby DNA is transferred through direct cell contact. In ordinary circumstances, transduction, conjugation, and transformation involve transfer of DNA between individual bacteria of the same species, but occasionally transfer may occur between individuals of different bacterial species and this may have significant consequences, such as the transfer of antibiotic resistance. In such cases, gene acquisition from other bacteria or the environment is called horizontal gene transfer and may be common under natural conditions.


Behaviour


Movement

Many bacteria are Motility, motile (able to move themselves) and do so using a variety of mechanisms. The best studied of these are Flagellum, flagella, long filaments that are turned by a motor at the base to generate propeller-like movement. The bacterial flagellum is made of about 20 proteins, with approximately another 30 proteins required for its regulation and assembly. The flagellum is a rotating structure driven by a reversible motor at the base that uses the
electrochemical gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and the ...
across the membrane for power. Bacteria can use flagella in different ways to generate different kinds of movement. Many bacteria (such as ''Escherichia coli, E. coli'') have two distinct modes of movement: forward movement (swimming) and tumbling. The tumbling allows them to reorient and makes their movement a three-dimensional random walk. Bacterial species differ in the number and arrangement of flagella on their surface; some have a single flagellum (''Flagellum#Flagellar arrangement schemes, monotrichous''), a flagellum at each end (''Flagellum#Flagellar arrangement schemes, amphitrichous''), clusters of flagella at the poles of the cell (''Flagellum#Flagellar arrangement schemes, lophotrichous''), while others have flagella distributed over the entire surface of the cell (''Flagellum#Flagellar arrangement schemes, peritrichous''). The flagella of a unique group of bacteria, the spirochaetes, are found between two membranes in the periplasmic space. They have a distinctive helix, helical body that twists about as it moves. Two other types of bacterial motion are called twitching motility that relies on a structure called the pilus#type iv pili, type IV pilus, and Bacterial gliding, gliding motility, that uses other mechanisms. In twitching motility, the rod-like pilus extends out from the cell, binds some substrate, and then retracts, pulling the cell forward. Motile bacteria are attracted or repelled by certain stimulus (physiology), stimuli in behaviours called taxis, taxes: these include chemotaxis, phototaxis, taxis, energy taxis, and magnetotaxis. In one peculiar group, the myxobacteria, individual bacteria move together to form waves of cells that then differentiate to form fruiting bodies containing spores. The myxobacteria move only when on solid surfaces, unlike ''E. coli'', which is motile in liquid or solid media. Several ''Listeria'' and ''Shigella'' species move inside host cells by usurping the
cytoskeleton 300px, The eukaryotic cytoskeleton. Actin filaments are shown in red, and microtubules composed of beta tubulin are in green. The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all ...
, which is normally used to move
organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the su ...
s inside the cell. By promoting actin biopolymer, polymerisation at one pole of their cells, they can form a kind of tail that pushes them through the host cell's cytoplasm.


Communication

A few bacteria have chemical systems that generate light. This bioluminescence often occurs in bacteria that live in association with fish, and the light probably serves to attract fish or other large animals. Bacteria often function as multicellular aggregates known as biofilms, exchanging a variety of molecular signals for Cell signaling, inter-cell communication, and engaging in coordinated multicellular behaviour. The communal benefits of multicellular cooperation include a cellular division of labour, accessing resources that cannot effectively be used by single cells, collectively defending against antagonists, and optimising population survival by differentiating into distinct cell types. For example, bacteria in biofilms can have more than 500 times increased resistance to Bactericide, antibacterial agents than individual "planktonic" bacteria of the same species. One type of inter-cellular communication by a molecular signal is called
quorum sensingIn biology, quorum sensing (or quorum signalling) is the ability to detect and respond to cell population density by gene regulation. As one example, quorum sensing (QS) enables bacteria to restrict the expression of specific genes to the high cell d ...
, which serves the purpose of determining whether there is a local population density that is sufficiently high that it is productive to invest in processes that are only successful if large numbers of similar organisms behave similarly, as in excreting digestive enzymes or emitting light. Quorum sensing allows bacteria to coordinate gene expression, and enables them to produce, release and detect autoinducers or pheromones which accumulate with the growth in cell population.


Classification and identification

Scientific classification, Classification seeks to describe the diversity of bacterial species by naming and grouping organisms based on similarities. Bacteria can be classified on the basis of cell structure, Cell metabolism, cellular metabolism or on differences in cell components, such as
DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, g ...
, fatty acids, pigments, antigens and quinones. While these schemes allowed the identification and classification of bacterial strains, it was unclear whether these differences represented variation between distinct species or between strains of the same species. This uncertainty was due to the lack of distinctive structures in most bacteria, as well as lateral gene transfer between unrelated species. Due to lateral gene transfer, some closely related bacteria can have very different morphologies and metabolisms. To overcome this uncertainty, modern bacterial classification emphasises molecular systematics, using genetic techniques such as guanine cytosine GC-content, ratio determination, genome-genome hybridisation, as well as DNA sequencing, sequencing genes that have not undergone extensive lateral gene transfer, such as the ribosomal DNA, rRNA gene. Classification of bacteria is determined by publication in the International Journal of Systematic Bacteriology, and Bergey's Manual of Systematic Bacteriology. The International Committee on Systematic Bacteriology (ICSB) maintains international rules for the naming of bacteria and taxonomic categories and for the ranking of them in the International Code of Nomenclature of Bacteria. The term "bacteria" was traditionally applied to all microscopic, single-cell prokaryotes. However, molecular systematics showed prokaryotic life to consist of two separate domain (biology), domains, originally called ''Eubacteria'' and ''Archaebacteria'', but now called ''Bacteria'' and ''
Archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
'' that evolved independently from an ancient common ancestor. The archaea and eukaryotes are more closely related to each other than either is to the bacteria. These two domains, along with Eukarya, are the basis of the three-domain system, which is currently the most widely used classification system in microbiology. However, due to the relatively recent introduction of molecular systematics and a rapid increase in the number of genome sequences that are available, bacterial classification remains a changing and expanding field. For example, Thomas Cavalier-Smith, Cavalier-Smith argued that the Archaea and Eukaryotes evolved from Gram-positive bacteria. The identification of bacteria in the laboratory is particularly relevant in medicine, where the correct treatment is determined by the bacterial species causing an infection. Consequently, the need to identify human pathogens was a major impetus for the development of techniques to identify bacteria. The ''
Gram stain Gram stain or Gram staining, also called Gram's method, is a method of staining used to distinguish and classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. The name comes from the Danish bacteriolo ...
'', developed in 1884 by Hans Christian Gram, characterises bacteria based on the structural characteristics of their cell walls. The thick layers of peptidoglycan in the "Gram-positive" cell wall stain purple, while the thin "Gram-negative" cell wall appears pink. By combining morphology and Gram-staining, most bacteria can be classified as belonging to one of four groups (Gram-positive cocci, Gram-positive bacilli, Gram-negative cocci and Gram-negative bacilli). Some organisms are best identified by stains other than the Gram stain, particularly mycobacteria or ''Nocardia'', which show acid-fastness on Ziehl-Neelsen stain, Ziehl–Neelsen or similar stains. Other organisms may need to be identified by their growth in special media, or by other techniques, such as serology. Microbiological culture, Culture techniques are designed to promote the growth and identify particular bacteria, while restricting the growth of the other bacteria in the sample. Often these techniques are designed for specific specimens; for example, a sputum sample will be treated to identify organisms that cause pneumonia, while feces, stool specimens are cultured on selective media to identify organisms that cause diarrhea, diarrhoea, while preventing growth of non-pathogenic bacteria. Specimens that are normally sterile, such as blood, urine or cerebrospinal fluid, spinal fluid, are cultured under conditions designed to grow all possible organisms. Once a pathogenic organism has been isolated, it can be further characterised by its morphology, growth patterns (such as aerobic organism, aerobic or anaerobic organism, anaerobic growth), hemolysis (microbiology), patterns of hemolysis, and staining. As with bacterial classification, identification of bacteria is increasingly using molecular methods. Diagnostics using DNA-based tools, such as polymerase chain reaction, are increasingly popular due to their specificity and speed, compared to culture-based methods. These methods also allow the detection and identification of "viable but nonculturable" cells that are metabolically active but non-dividing. However, even using these improved methods, the total number of bacterial species is not known and cannot even be estimated with any certainty. Following present classification, there are a little less than 9,300 known species of prokaryotes, which includes bacteria and archaea; but attempts to estimate the true number of bacterial diversity have ranged from 107 to 109 total species—and even these diverse estimates may be off by many orders of magnitude.


Phylogenetic tree

According to the phylogenetic analysis of Zhu (2019), the relationships could be the following:Zhu, Q., Mai, U., Pfeiffer, W. ''et al.'' (2019)
Phylogenomics of 10,575 genomes reveals evolutionary proximity between domains Bacteria and Archaea
. ''Nature Communications'', 10: 5477


Interactions with other organisms

Despite their apparent simplicity, bacteria can form complex associations with other organisms. These symbiosis, symbiotic associations can be divided into parasitism, Mutualism (biology), mutualism and commensalism. Due to their small size, commensal bacteria are ubiquitous and grow on animals and plants exactly as they will grow on any other surface. However, their growth can be increased by warmth and Perspiration, sweat, and large populations of these organisms in humans are the cause of body odor, body odour.


Predators

Some species of bacteria kill and then consume other microorganisms, these species are called ''predatory bacteria''. These include organisms such as ''Myxococcus xanthus'', which forms swarms of cells that kill and digest any bacteria they encounter. Other bacterial predators either attach to their prey in order to digest them and absorb nutrients, such as ''Vampirovibrio chlorellavorus'', or invade another cell and multiply inside the cytosol, such as ''Daptobacter''. These predatory bacteria are thought to have evolved from Detritivore, saprophages that consumed dead microorganisms, through adaptations that allowed them to entrap and kill other organisms.


Mutualists

Certain bacteria form close spatial associations that are essential for their survival. One such mutualistic association, called interspecies hydrogen transfer, occurs between clusters of anaerobic bacteria that consume organic acids, such as butyric acid or propionic acid, and produce hydrogen, and methanogenic Archaea that consume hydrogen. The bacteria in this association are unable to consume the organic acids as this reaction produces hydrogen that accumulates in their surroundings. Only the intimate association with the hydrogen-consuming Archaea keeps the hydrogen concentration low enough to allow the bacteria to grow. In soil, microorganisms that reside in the Rhizosphere (ecology), rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) carry out nitrogen fixation, converting nitrogen gas to nitrogenous compounds. This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as symbionts Bacteria in the human body, in humans and other organisms. For example, the presence of over 1,000 bacterial species in the normal human gut flora of the intestines can contribute to gut immunity, synthesise
vitamin A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an essential micronutrient which an organism needs in small quantities for the proper functioning of its metabolism. Essential nutrients ...
s, such as folic acid, vitamin K and biotin, convert Milk protein, sugars to lactic acid (see ''Lactobacillus''), as well as fermenting complex undigestible carbohydrates. The presence of this gut flora also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion) and these beneficial bacteria are consequently sold as probiotic dietary supplements.


Pathogens

If bacteria form a parasitic association with other organisms, they are classed as pathogens. Pathogenic bacteria are a major cause of human death and disease and cause infections such as tetanus (caused by ''Clostridium tetani''), typhoid fever, diphtheria,
syphilis Syphilis is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, and tert ...
,
cholera Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and mu ...

cholera
, foodborne illness,
leprosy Leprosy, also known as Hansen's disease (HD), is a long-term infection by the bacteria ''Mycobacterium leprae'' or ''Mycobacterium lepromatosis''. Infection can lead to damage of the nerves, respiratory tract, skin, and eyes. This nerve damage ma ...

leprosy
(caused by ''Micobacterium leprae)'' and tuberculosis (caused by ''Mycobacterium tuberculosis''). A pathogenic cause for a known medical disease may only be discovered many years later, as was the case with ''Helicobacter pylori'' and Timeline of peptic ulcer disease and Helicobacter pylori, peptic ulcer disease. Bacterial diseases are also important in agriculture, with bacteria causing leaf spot, fire blight and Wilting, wilts in plants, as well as Johne's disease, Mastitis in dairy cattle, mastitis, salmonellosis, salmonella and
anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day to over two months after the infection is contracted. The skin form ...

anthrax
in farm animals. Each species of pathogen has a characteristic spectrum of interactions with its human host (biology), hosts. Some organisms, such as ''
Staphylococcus ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...
'' or ''
Streptococcus ''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Firmicutes. Cell division in streptococci occurs ...
'', can cause skin infections, pneumonia, meningitis and sepsis, a systemic Inflammation, inflammatory response producing shock (circulatory), shock, massive vasodilator, vasodilation and death. Yet these organisms are also part of the normal human flora and usually exist on the skin or in the nose without causing any disease at all. Other organisms invariably cause disease in humans, such as the Rickettsia, which are obligate intracellular parasites able to grow and reproduce only within the cells of other organisms. One species of Rickettsia causes typhus, while another causes Rocky Mountain spotted fever. ''Chlamydia (bacterium), Chlamydia'', another phylum of obligate intracellular parasites, contains species that can cause pneumonia or urinary tract infection and may be involved in coronary heart disease. Finally, some species, such as ''
Pseudomonas aeruginosa ''Pseudomonas aeruginosa'' is a common encapsulated, Gram-negative, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. aeruginosa'' is a multidrug resistant path ...

Pseudomonas aeruginosa
'', ''Burkholderia cenocepacia'', and ''Mycobacterium avium complex, Mycobacterium avium'', are opportunistic infection, opportunistic pathogens and cause disease mainly in people suffering from immunosuppression or cystic fibrosis. Bacterial infections may be treated with antibiotics, which are classified as Bactericide, bacteriocidal if they kill bacteria or bacteriostatic if they just prevent bacterial growth. There are many types of antibiotics, and each class enzyme inhibitor, inhibits a process that is different in the pathogen from that found in the host. An example of how antibiotics produce selective toxicity are chloramphenicol and puromycin, which inhibit the bacterial
ribosome Ribosomes () are macromolecular machines, found within all living cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to f ...
, but not the structurally different eukaryotic ribosome. Antibiotics are used both in treating human disease and in intensive farming to promote animal growth, where they may be contributing to the rapid development of
antibiotic resistance Antimicrobial resistance (AMR or AR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. The term antibiotic resistance (AR or ABR) is a subset of AMR, as it applies to bacteria that become resistant to ant ...

antibiotic resistance
in bacterial populations. Infections can be prevented by antiseptic measures such as sterilising the skin prior to piercing it with the needle of a syringe, and by proper care of indwelling catheters. Surgical and dental instruments are also sterilization (microbiology), sterilised to prevent contamination by bacteria. Disinfectants such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection.


Significance in technology and industry

Bacteria, often lactic acid bacteria, such as ''Lactobacillus'' and ''Lactococcus'', in combination with yeasts and Mold (fungus), moulds, have been used for thousands of years in the preparation of fermentation (food), fermented foods, such as
cheese Cheese is a dairy product, derived from milk and produced in wide ranges of flavors, textures and forms by coagulation of the milk protein casein. It comprises proteins and fat from milk, usually the milk of cows, buffalo, goats, or sheep. Du ...

cheese
, Pickling, pickles, soy sauce, sauerkraut, vinegar, wine and
yogurt Yogurt (; , from tr, yoğurt, arm, յոգուրտ) also spelled yoghurt, yogourt or yoghourt, is a food produced by bacterial fermentation of milk. The bacteria used to make yogurt are known as ''yogurt cultures''. Fermentation of sugars in th ...
. The ability of bacteria to degrade a variety of organic compounds is remarkable and has been used in waste processing and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up
oil spill An oil spill is the release of a liquid petroleum hydrocarbon into the environment, especially the marine ecosystem, due to human activity, and is a form of pollution. The term is usually given to marine oil spills, where oil is released into the ...
s. Fertiliser was added to some of the beaches in Prince William Sound in an attempt to promote the growth of these naturally occurring bacteria after the 1989 Exxon Valdez oil spill, ''Exxon Valdez'' oil spill. These efforts were effective on beaches that were not too thickly covered in oil. Bacteria are also used for the bioremediation of industrial toxic wastes. In the chemical industry, bacteria are most important in the production of enantiomerically pure chemicals for use as pharmaceutical company, pharmaceuticals or agrichemicals. Bacteria can also be used in the place of pesticides in the biological pest control. This commonly involves ''Bacillus thuringiensis'' (also called BT), a Gram-positive, soil dwelling bacterium. Subspecies of this bacteria are used as a Lepidopteran-specific insecticides under trade names such as Dipel and Thuricide. Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humans, wildlife, pollinators and most other beneficial insects. Because of their ability to quickly grow and the relative ease with which they can be manipulated, bacteria are the workhorses for the fields of molecular biology, genetics and biochemistry. By making mutations in bacterial DNA and examining the resulting phenotypes, scientists can determine the function of genes,
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules k ...
s and metabolic pathways in bacteria, then apply this knowledge to more complex organisms. This aim of understanding the biochemistry of a cell reaches its most complex expression in the synthesis of huge amounts of enzyme kinetics, enzyme kinetic and gene expression data into mathematical models of entire organisms. This is achievable in some well-studied bacteria, with models of ''Escherichia coli'' metabolism now being produced and tested. This understanding of bacterial metabolism and genetics allows the use of biotechnology to bioengineering, bioengineer bacteria for the production of therapeutic proteins, such as insulin, growth factors, or antibody, antibodies. Because of their importance for research in general, samples of bacterial strains are isolated and preserved in Biological Resource Centers. This ensures the availability of the strain to scientists worldwide.


History of bacteriology

Bacteria were first observed by the Dutch microscopist Antonie van Leeuwenhoek in 1676, using a single-lens microscope of his own design. He then published his observations in a series of letters to the Royal Society of London. Bacteria were Leeuwenhoek's most remarkable microscopic discovery. They were just at the limit of what his simple lenses could make out and, in one of the most striking hiatuses in the history of science, no one else would see them again for over a century. His observations had also included protozoans which he called animalcules, and his findings were looked at again in the light of the more recent findings of cell theory. Christian Gottfried Ehrenberg introduced the word "bacterium" in 1828. In fact, his ''Bacterium (genus), Bacterium'' was a genus that contained non-spore-forming rod-shaped bacteria, as opposed to ''Bacillus'', a genus of spore-forming rod-shaped bacteria defined by Ehrenberg in 1835. Louis Pasteur demonstrated in 1859 that the growth of microorganisms causes the fermentation (food), fermentation process, and that this growth is not due to spontaneous generation (yeasts and Mold (fungus), molds, commonly associated with fermentation, are not bacteria, but rather
fungi A fungus (plural: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, which is separa ...
). Along with his contemporary Robert Koch, Pasteur was an early advocate of the germ theory of disease. Robert Koch, a pioneer in medical microbiology, worked on
cholera Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and mu ...

cholera
,
anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day to over two months after the infection is contracted. The skin form ...

anthrax
and tuberculosis. In his research into tuberculosis Koch finally proved the germ theory, for which he received a Nobel Prize in Physiology or Medicine, Nobel Prize in 1905. In Koch's postulates, he set out criteria to test if an organism is the cause of a disease, and these postulates are still used today. Ferdinand Cohn is said to be a founder of bacteriology, studying bacteria from 1870. Cohn was the first to classify bacteria based on their morphology. Though it was known in the nineteenth century that bacteria are the cause of many diseases, no effective antiseptic, antibacterial treatments were available. In 1910, Paul Ehrlich developed the first antibiotic, by changing dyes that selectively stained ''Treponema pallidum''—the spirochaete that causes
syphilis Syphilis is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, and tert ...
—into compounds that selectively killed the pathogen. Ehrlich had been awarded a 1908 Nobel Prize for his work on immunology, and pioneered the use of stains to detect and identify bacteria, with his work being the basis of the
Gram stain Gram stain or Gram staining, also called Gram's method, is a method of staining used to distinguish and classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. The name comes from the Danish bacteriolo ...
and the Ziehl–Neelsen stain. A major step forward in the study of bacteria came in 1977 when Carl Woese recognised that
archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archae ...
have a separate line of evolutionary descent from bacteria. This new phylogenetic Taxonomy (biology), taxonomy depended on the sequencing of 16S ribosomal RNA, and divided prokaryotes into two evolutionary domains, as part of the three-domain system.


See also

* Bacteriotherapy * Genetically modified bacteria * List of bacterial orders * Panspermia * Polysaccharide encapsulated bacteria * Psychrotrophic bacteria * Segrosome


References


Bibliography

*


External links


On-line text book on bacteriology
{{Authority control Bacteria, Bacteriology Domains (biology), Bacteria 1670s in science 1680s in science 1670s in the Dutch Republic 1680s in the Dutch Republic Biology and natural history in the Dutch Republic Microscopic discoveries by Antonie van Leeuwenhoek