Eubacteria Woese & Fox, 1977
Bacteria (/bækˈtɪəriə/ ( listen); common noun bacteria,
singular bacterium) constitute a large domain of prokaryotic
microorganisms. Typically a few micrometres in length, bacteria have a
number of shapes, ranging from spheres to rods and spirals. Bacteria
were among the first life forms to appear on Earth, and are present in
most of its habitats.
Bacteria inhabit soil, water, acidic hot
springs, radioactive waste, and the deep portions of Earth's crust.
Bacteria also live in symbiotic and parasitic relationships with
plants and animals. Most bacteria have not been characterised, and
only about half of the bacterial phyla have species that can be grown
in the laboratory. The study of bacteria is known as bacteriology,
a branch of microbiology.
There are typically 40 million bacterial cells in a gram of soil and a
million bacterial cells in a millilitre of fresh water. There are
approximately 5×1030 bacteria on Earth, forming a biomass which
exceeds that of all plants and animals.
Bacteria are vital in many
stages of the nutrient cycle by recycling nutrients such as the
fixation of nitrogen from the atmosphere. The nutrient cycle includes
the decomposition of dead bodies and bacteria are responsible for the
putrefaction stage in this process. In the biological communities
surrounding hydrothermal vents and cold seeps, extremophile bacteria
provide the nutrients needed to sustain life by converting dissolved
compounds, such as hydrogen sulphide and methane, to energy. In March
2013, data reported by researchers in October 2012, was published. It
was suggested that bacteria thrive in the Mariana Trench, which with a
depth of up to 11 kilometres is the deepest known part of the
oceans. Other researchers reported related studies that microbes
thrive inside rocks up to 580 metres below the sea floor under 2.6
kilometres of ocean off the coast of the northwestern United
States. According to one of the researchers, "You can find
microbes everywhere—they're extremely adaptable to conditions, and
survive wherever they are."
The famous notion that bacterial cells in the human body outnumber
human cells by a factor of 10:1 has been debunked. There are
approximately 39 trillion bacterial cells in the human microbiota as
personified by a "reference" 70 kg male 170 cm tall, whereas
there are 30 trillion human cells in the body. This means that
although they do have the upper hand in actual numbers, it is only by
30%, and not 900%.
The largest number exist in the gut flora, and a large number on the
skin. The vast majority of the bacteria in the body are rendered
harmless by the protective effects of the immune system, though many
are beneficial particularly in the gut flora. However several species
of bacteria are pathogenic and cause infectious diseases, including
cholera, syphilis, anthrax, leprosy, and bubonic plague. The most
common fatal bacterial diseases are respiratory infections, with
tuberculosis alone killing about 2 million people per year, mostly in
sub-Saharan Africa. In developed countries, antibiotics are used
to treat bacterial infections and are also used in farming, making
antibiotic resistance a growing problem. In industry, bacteria are
important in sewage treatment and the breakdown of oil spills, the
production of cheese and yogurt through fermentation, the recovery of
gold, palladium, copper and other metals in the mining sector, as
well as in biotechnology, and the manufacture of antibiotics and other
Once regarded as plants constituting the class Schizomycetes, bacteria
are now classified as prokaryotes. Unlike cells of animals and other
eukaryotes, bacterial cells do not contain a nucleus and rarely
harbour membrane-bound organelles. Although the term bacteria
traditionally included all prokaryotes, the scientific classification
changed after the discovery in the 1990s that prokaryotes consist of
two very different groups of organisms that evolved from an ancient
common ancestor. These evolutionary domains are called
2 Origin and early evolution
4 Cellular structure
4.1 Intracellular structures
4.2 Extracellular structures
6 Growth and reproduction
9 Classification and identification
10 Interactions with other organisms
11 Significance in technology and industry
12 History of bacteriology
13 See also
15 Further reading
16 External links
view • discuss • edit
Earliest sexual reproduction
Axis scale: million years
Orange labels: ice ages.
Human timeline and Nature timeline
The word bacteria is the plural of the
New Latin bacterium, which is
the latinisation of the Greek βακτήριον (bakterion), the
diminutive of βακτηρία (bakteria), meaning "staff, cane",
because the first ones to be discovered were rod-shaped.
Origin and early evolution
Timeline of evolution
Timeline of evolution and Evolutionary history of
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 fossils exist, such as stromatolites, their lack of
distinctive morphology 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, and these studies indicate that
bacteria diverged first from the archaeal/eukaryotic lineage. The
most recent common ancestor of bacteria and archaea was probably a
hyperthermophile that lived about 2.5 billion–3.2 billion years
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
associations with the ancestors of eukaryotic cells, which were
themselves possibly related to the Archaea. This involved the
engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts
to form either mitochondria or hydrogenosomes, which are still found
in all known Eukarya (sometimes in highly reduced form, e.g. in
ancient "amitochondrial" protozoa). Later, some eukaryotes that
already contained mitochondria also engulfed cyanobacteria-like
organisms, leading to the formation of chloroplasts in algae and
plants. This is known as secondary endosymbiosis.
Bacterial cell structure
Bacterial cell structure § Cell morphology
Bacteria display many cell morphologies and arrangements
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 micrometres in
length. However, a few species are visible to the unaided eye—for
Thiomargarita namibiensis is up to half a millimetre long
Epulopiscium fishelsoni reaches 0.7 mm. Among the
smallest bacteria are members of the genus Mycoplasma, which measure
only 0.3 micrometres, as small as the largest viruses. Some
bacteria may be even smaller, but these ultramicrobacteria are not
Most bacterial species are either spherical, called cocci (sing.
coccus, from Greek kókkos, grain, seed), or rod-shaped, called
bacilli (sing. bacillus, from
Latin baculus, stick). Some
bacteria, called vibrio, are shaped like slightly curved rods or
comma-shaped; others can be spiral-shaped, called spirilla, or tightly
coiled, called 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 and
cytoskeleton, and is important because it can influence the ability of
bacteria to acquire nutrients, attach to surfaces, swim through
liquids and escape predators.
The range of sizes shown by prokaryotes, relative to those of other
organisms and biomolecules.
Many bacterial species exist simply as single cells, others associate
in characteristic patterns:
Neisseria form diploids (pairs),
Streptococcus form chains, and
Staphylococcus group together in "bunch
of grapes" clusters.
Bacteria can also group to form larger
multicellular structures, such as the elongated filaments of
Actinobacteria, the aggregates of Myxobacteria, and the complex hyphae
of Streptomyces. These multicellular structures are often only
seen in certain conditions. For example, when starved of amino acids,
Myxobacteria detect surrounding cells in a process known as quorum
sensing, 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
biofilms, and larger formations known as microbial mats. 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, protists and archaea.
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 devices, and bacteria protected within biofilms are much
harder to kill than individual isolated bacteria.
Further information: Bacterial cell structure
Structure and contents of a typical gram-positive bacterial cell (seen
by the fact that only one cell membrane is present).
The bacterial cell is surrounded by a cell membrane which is made
primarily of phospholipids. This membrane encloses the contents of the
cell and acts as a barrier to hold nutrients, proteins and other
essential components of the cytoplasm within the cell. Unlike
eukaryotic cells, bacteria usually lack large membrane-bound
structures in their cytoplasm such as a nucleus, mitochondria,
chloroplasts 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. Additionally, bacteria have a multi-component
cytoskeleton to control the localisation of proteins and nucleic acids
within the cell, and to manage the process of cell
Many important biochemical reactions, such as energy generation, occur
due to concentration gradients across membranes, creating a potential
difference analogous to a battery. The general lack of internal
membranes in bacteria means these reactions, such as electron
transport, occur across the cell membrane between the cytoplasm and
the outside of the cell or periplasm. 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 chlorosomes in green sulfur bacteria.
An electron micrograph of
Halothiobacillus neapolitanus cells with
carboxysomes inside, with arrows highlighting visible carboxysomes.
Scale bars indicate 100 nm.
Most bacteria do not have a membrane-bound nucleus, and their genetic
material is typically a single circular bacterial chromosome of DNA
located in the cytoplasm in an irregularly shaped body called the
nucleoid. The nucleoid contains the chromosome with its associated
proteins and RNA. Like all living organisms, bacteria contain
ribosomes for the production of proteins, but the structure of the
bacterial ribosome is different from that of eukaryotes and
Some bacteria produce intracellular nutrient storage granules, such as
glycogen, polyphosphate, sulfur or
polyhydroxyalkanoates. Certain bacterial species, such as the
photosynthetic Cyanobacteria, 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.
Further information: Cell envelope
Around the outside of the cell membrane is the cell wall. Bacterial
cell walls are made of peptidoglycan (called "murein" in older
sources), which is made from polysaccharide chains cross-linked by
peptides containing D-amino acids. Bacterial cell walls are
different from the cell walls of plants and fungi, which are made of
cellulose and 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 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
Gram-positive and Gram-negative. The names originate
from the reaction of cells to the Gram stain, 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 acids. In contrast, Gram-negative
bacteria have a relatively thin cell wall consisting of a few layers
of peptidoglycan surrounded by a second lipid membrane containing
lipopolysaccharides and lipoproteins. Most bacteria have the
Gram-negative cell wall, and only the
Firmicutes and Actinobacteria
(previously known as the low G+C and high G+C gram-positive bacteria,
respectively) have the alternative
These differences in structure can produce differences in antibiotic
susceptibility; for instance, vancomycin can kill only Gram-positive
bacteria and is ineffective against
Gram-negative pathogens, such as
Haemophilus influenzae or 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 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-layer 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 diffusion barrier. S-layers have diverse but mostly
poorly understood functions, but are known to act as virulence factors
Campylobacter and contain surface enzymes in Bacillus
Helicobacter pylori electron micrograph, showing multiple flagella on
the cell surface
Flagella are rigid protein structures, about 20 nanometres in
diameter and up to 20 micrometres in length, that are used for
motility. Flagella are driven by the energy released by the transfer
of ions down an electrochemical gradient 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.
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
between bacterial cells in a process called conjugation where they are
called conjugation pili or sex pili (see bacterial genetics,
below). They can also generate movement where they are called type
Glycocalyx 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
capsule. These structures can protect cells from engulfment by
eukaryotic cells such as macrophages (part of the human immune
system). 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
Further information: Endospore
Bacillus anthracis (stained purple) growing in cerebrospinal fluid
Certain genera of
Gram-positive bacteria, such as Bacillus,
Clostridium, Sporohalobacter, Anaerobacter, and 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
DNA and ribosomes surrounded by a cortex layer and protected
by a multilayer rigid coat composed of peptidoglycan and a variety of
Endospores show no detectable metabolism and can survive extreme
physical and chemical stresses, such as high levels of UV light, 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 vacuum and radiation in space.
Endospore-forming bacteria can also cause disease: for example,
anthrax can be contracted by the inhalation of
endospores, and contamination of deep puncture wounds with Clostridium
tetani endospores causes tetanus.
Further information: Microbial metabolism
Bacteria exhibit an extremely wide variety of metabolic types. The
distribution of metabolic traits within a group of bacteria has
traditionally been used to define their taxonomy, but these traits
often do not correspond with modern genetic classifications.
Bacterial metabolism is classified into nutritional groups on the
basis of three major criteria: the source of 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 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
Bacteria that use inorganic compounds such as hydrogren,
carbon monoxide, or ammonia as 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 carbon, called
heterotrophy. Others such as cyanobacteria and some purple bacteria
are autotrophic, meaning that they obtain cellular carbon by fixing
carbon dioxide. In unusual circumstances, the gas methane can be
used by methanotrophic bacteria as both a source of electrons and a
substrate for carbon anabolism.
Nutritional types in bacterial metabolism
Source of energy
Source of carbon
Organic compounds (photoheterotrophs) or carbon fixation
Cyanobacteria, Green sulfur bacteria, Chloroflexi, or Purple
Organic compounds (lithoheterotrophs) or carbon fixation
Thermodesulfobacteria, Hydrogenophilaceae, or Nitrospirae
Organic compounds (chemoheterotrophs) or carbon fixation
Clostridium or Enterobacteriaceae
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 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 (methyl- and
dimethylmercury) in the environment. Non-respiratory anaerobes use
fermentation 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
Many bacteria reproduce through binary fission, which is compared to
mitosis and meiosis in this image.
Further information: Bacterial growth
Unlike in multicellular organisms, increases in cell size (cell
growth) and reproduction by cell division are tightly linked in
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 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 and aerial hyphae formation by Streptomyces, or budding.
Budding involves a cell forming a protrusion that breaks away and
produces a daughter cell.
A colony of Escherichia coli
In the laboratory, bacteria are usually grown using solid or liquid
media. Solid growth media, such as agar plates, are used to isolate
pure cultures of a bacterial strain. However, liquid growth media are
used when 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 (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
inhibit the growth of competing microorganisms. In nature, many
organisms live in communities (e.g., biofilms) 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 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
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 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 expression of
genes involved in
DNA repair, antioxidant metabolism and nutrient
transport. The final phase is the death phase where the bacteria
run out of nutrients and die.
Main article: Bacterial genetics
Most bacteria have a single circular chromosome that can range in size
from only 160,000 base pairs in the endosymbiotic bacteria 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
Borrelia species contain a single
linear chromosome, while some
Vibrio species contain more
than one chromosome.
Bacteria can also contain plasmids, small
extra-chromosomal DNAs that may contain genes for various useful
functions such as antibiotic resistance, metabolic capabilities, or
various 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
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. However, all bacteria can evolve by
selection on changes to their genetic material
DNA caused by genetic
recombination or mutations. Mutations come from errors made during the
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. Many
bacteria can 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, when the integration of a bacteriophage introduces
DNA into the chromosome. Many types of bacteriophage exist,
some simply infect and lyse their 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
RNA interference. The third method of gene transfer
is conjugation, 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.
Further information: Chemotaxis, Flagellum, and Pilus
Transmission electron micrograph of
Desulfovibrio vulgaris showing a
single flagellum at one end of the cell. Scale bar is 0.5 micrometers
Many bacteria are motile and can move using a variety of mechanisms.
The best studied of these are 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 across the membrane for
The different arrangements of bacterial flagella: A-Monotrichous;
B-Lophotrichous; C-Amphitrichous; D-Peritrichous
Bacteria can use flagella in different ways to generate different
kinds of movement. Many bacteria (such as 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 (monotrichous), a flagellum at each end
(amphitrichous), clusters of flagella at the poles of the cell
(lophotrichous), while others have flagella distributed over the
entire surface of the cell (peritrichous). The flagella of a unique
group of bacteria, the spirochaetes, are found between two membranes
in the periplasmic space. They have a distinctive helical body that
twists about as it moves.
Two other types of bacterial motion, called twitching motility and
gliding motility, rely on a structure called the type IV pilus.
In these types of motility, the rod-like pilus extends out from the
cell, binds some substrate, and then retracts, pulling the cell
Motile bacteria are attracted or repelled by certain stimuli in
behaviours called taxes: these include chemotaxis, phototaxis, 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.
Shigella species move inside host cells by
usurping the cytoskeleton, which is normally used to move organelles
inside the cell. By promoting actin polymerisation at one pole of
their cells, they can form a kind of tail that pushes them through the
host cell's cytoplasm.
Prokaryote § Sociality
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
Bacteria often function as multicellular aggregates known as biofilms,
exchanging a variety of molecular signals for inter-cell
communication, and engaging in coordinated multicellular
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 antibacterial agents than individual
"planktonic" bacteria of the same species.
One type of inter-cellular communication by a molecular signal is
called quorum sensing, 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
Main article: Bacterial taxonomy
Further information: Scientific classification, Systematics, Bacterial
phyla, and Clinical pathology
Streptococcus mutans visualised with a Gram stain
Phylogenetic tree showing the diversity of bacteria, compared to other
organisms. Eukaryotes are coloured red, archaea green and
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, cellular metabolism or on
differences in cell components, such as DNA, 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 ratio determination,
genome-genome hybridisation, as well as sequencing genes that have not
undergone extensive lateral gene transfer, such as the r
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
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 domains, originally called
Eubacteria and Archaebacteria, but now called
Bacteria and Archaea
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, a few biologists argue 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, 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 or similar stains.
Other organisms may need to be identified by their growth in special
media, or by other techniques, such as serology.
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 stool specimens are
cultured on selective media to identify organisms that cause
diarrhoea, while preventing growth of non-pathogenic bacteria.
Specimens that are normally sterile, such as blood, urine or 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 or anaerobic growth), 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.
Interactions with other organisms
Overview of bacterial infections and main species involved.
Further information: Microbes in human culture
Despite their apparent simplicity, bacteria can form complex
associations with other organisms. These symbiotic associations can be
divided into parasitism, 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 sweat, and large populations of
these organisms in humans are the cause of body odour.
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 saprophages that consumed dead microorganisms, through
adaptations that allowed them to entrap and kill other organisms.
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
Archaea keeps the hydrogen concentration low enough
to allow the bacteria to grow.
In soil, microorganisms that reside in the 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 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 vitamins, such as folic acid, vitamin K
and biotin, convert 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
Main article: Pathogenic bacteria
Colour-enhanced scanning electron micrograph showing Salmonella
typhimurium (red) invading cultured human cells
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, typhoid
fever, diphtheria, syphilis, cholera, foodborne illness, leprosy and
tuberculosis. A pathogenic cause for a known medical disease may only
be discovered many years after, as was the case with Helicobacter
pylori and peptic ulcer disease. Bacterial diseases are also important
in agriculture, with bacteria causing leaf spot, fire blight and wilts
in plants, as well as Johne's disease, mastitis, salmonella and
anthrax in farm animals.
Each species of pathogen has a characteristic spectrum of interactions
with its human hosts. Some organisms, such as
Streptococcus, can cause skin infections, pneumonia, meningitis and
even overwhelming sepsis, a systemic inflammatory response producing
shock, massive 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
typhus, while another causes Rocky Mountain spotted fever. 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, Burkholderia cenocepacia, and Mycobacterium
avium, are 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 bacteriocidal if they kill bacteria, or bacteriostatic
if they just prevent bacterial growth. There are many types of
antibiotics and each class 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, 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
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 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
Further information: Economic importance of bacteria
Bacteria, often lactic acid bacteria, such as
Lactococcus, in combination with yeasts and moulds, have been used for
thousands of years in the preparation of fermented foods, such as
cheese, pickles, soy sauce, sauerkraut, vinegar, wine and
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 spills. Fertiliser was added to some of the
Prince William Sound
Prince William Sound in an attempt to promote the growth of
these naturally occurring bacteria after the 1989 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 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, enzymes 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 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 bioengineer bacteria for
the production of therapeutic proteins, such as insulin, growth
factors, or 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
History of bacteriology
For the history of microbiology, see Microbiology. For the history of
bacterial classification, see Bacterial taxonomy. For the natural
history of Bacteria, see Last universal common ancestor.
Antonie van Leeuwenhoek, the first microbiologist and the first person
to observe bacteria using a microscope.
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
Royal Society of London.
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
Christian Gottfried Ehrenberg
Christian Gottfried Ehrenberg introduced the word "bacterium" in
1828. In fact, his
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
Louis Pasteur demonstrated in 1859 that the growth of microorganisms
causes the fermentation process, and that this growth is not due to
spontaneous generation. (Yeasts and moulds, commonly associated with
fermentation, are not bacteria, but rather fungi.) 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,
anthrax and tuberculosis. In his research into tuberculosis Koch
finally proved the germ theory, for which he received a 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
Ferdinand Cohn is said to be a founder of bacteriology, studying
bacteria from 1870. Cohn was the first to classify bacteria based on
Though it was known in the nineteenth century that bacteria are the
cause of many diseases, no effective 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—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
and the Ziehl–Neelsen stain.
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Woese recognised that archaea have a separate line of evolutionary
descent from bacteria. This new phylogenetic taxonomy depended on
the sequencing of 16S ribosomal RNA, and divided prokaryotes into two
evolutionary domains, as part of the three-domain system.
Genetically modified bacteria
List of bacterial orders
Polysaccharide encapsulated bacteria
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Library resources about
Resources in your library
Resources in other libraries
Alcamo IE (2001). Fundamentals of microbiology. Boston: Jones and
Bartlett. ISBN 0-7637-1067-9.
Atlas RM (1995). Principles of microbiology. St. Louis: Mosby.
Martinko JM, Madigan MT (2005). Brock Biology of Microorganisms (11th
ed.). Englewood Cliffs, N.J: Prentice Hall.
Holt JC, Bergey DH (1994). Bergey's manual of determinative
bacteriology (9th ed.). Baltimore: Williams & Wilkins.
Hugenholtz P, Goebel BM, Pace NR (September 1998). "Impact of
culture-independent studies on the emerging phylogenetic view of
bacterial diversity". Journal of Bacteriology. 180 (18): 4765–74.
PMC 107498 . PMID 9733676.
Funke BR, Tortora GJ, Case CL (2004). Microbiology: an introduction
(8th ed.). San Francisco: Benjamin Cummings.
Ogunseitan OA (2005). Microbial Diversity: Form and Function in
Prokaryotes. Wiley-Blackwell. ISBN 978-1-4051-4448-3.
Shively JM (2006). Complex Intracellular Structures in Prokaryotes
Microbiology Monographs). Berlin: Springer.
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MicrobeWiki, an extensive wiki about bacteria and viruses
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Bacterial Nomenclature Up-To-Date from DSMZ
Genera of the domain Bacteria—list of
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Standing in Nomenclature
The largest bacteria
Tree of Life: Eubacteria
Videos of bacteria swimming and tumbling, use of optical tweezers and
Planet of the
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On-line text book on bacteriology
Animated guide to bacterial cell structure.
Bacteria Make Major Evolutionary Shift in the Lab
Online collaboration for bacterial taxonomy.
PATRIC, a Bioinformatics Resource Center for bacterial pathogens,
funded by NIAID
Chemotaxis Interactive Simulator—A web-app that uses
several simple algorithms to simulate bacterial chemotaxis.
Cell-Cell Communication in
Bacteria on-line lecture by Bonnie Bassler,
and TED: Discovering bacteria's amazing communication system
Sulfur-cycling fossil bacteria from the 1.8-Ga Duck Creek Formation
provide promising evidence of evolution's null hypothesis, Proceedings
of the National Academy of Sciences of the United States of
America. Summarised in: Scientists discover bacteria that
haven't evolved in more than 2 billion years,
Bacteria classification (phyla and orders)
Source: Bergey's Manual (2001–2012). Alternative views: Wikispecies.
Vaginal (In pregnancy)
Primary nutritional groups
Bacterial cellular morphologies
Cell wall: Peptidoglycan
Gram-positive bacteria only: Teichoic acid
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Microbial population biology
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Host microbe interactions in Caenorhabditis elegans
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International Census of Marine Microbes
Microbes in human culture
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Human Microbiome Project
Lines on the Antiquity of Microbes
Microbially induced sedimentary structure
Microbial dark matter
Physical factors affecting microbial life
Antonie van Leeuwenhoek
Microscopic discoveries 1
Spermatozoa (sperm cells)
Red blood cells
Crystals in gouty tophi
Microscopic discovery of microorganisms
History of biology
History of microbiology
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History of the microscope
Invention of the optical microscope
Timeline of microscope technology
Golden Age of Dutch science and technology
Science and technology in the Dutch Republic
Age of Reason
Clifford Dobell (Leeuwenhoek scholar)
Brian J. Ford (Leeuwenhoek scholar)
Regnier de Graaf
Robert Hooke (author of Micrographia)
Antoni van Leeuwenhoek Ziekenhuis
List of people considered father or mother of a scientific field
1 First observed, described, and studied by van Leeuwenhoek.