A
bacterium, despite its simplicity, contains a well-developed
cell structure which is responsible for some of its unique
biological structures and pathogenicity. Many structural features are unique to bacteria, and are not found among
archaea
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
or
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s. Because of the simplicity of bacteria relative to larger organisms and the ease with which they can be manipulated experimentally, the cell structure of bacteria has been well studied, revealing many
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, ...
principles that have been subsequently applied to other organisms.
Cell morphology

Perhaps the most elemental structural property of
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
is their
morphology (shape). Typical examples include:
*
coccus (circle or spherical)
*
bacillus (rod-like)
*
coccobacillus
Bacterial cellular morphologies are the shapes that are characteristic of various types of bacteria and often key to their identification. Their direct examination under a light microscope enables the classification of these bacteria (and archae ...
(between a sphere and a rod)
*
spiral (corkscrew-like)
*
filamentous (elongated)
Cell shape is generally characteristic of a given bacterial species, but can vary depending on growth conditions. Some bacteria have complex life cycles involving the production of stalks and appendages (e.g. ''
Caulobacter'') and some produce elaborate structures bearing reproductive spores (e.g. ''
Myxococcus'', ''
Streptomyces'').
Bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
generally form distinctive cell morphologies when examined by
light microscopy and distinct
colony morphologies when grown on
Petri plates.
Perhaps the most obvious structural characteristic of
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
is (with some exceptions) their small size. For example, ''
Escherichia coli
''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
'' cells, an "average" sized bacterium, are about 2 μm (
micrometre
The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...
s) long and 0.5 μm in diameter, with a cell volume of 0.6–0.7 μm
3. This corresponds to a wet mass of about 1
picogram (pg), assuming that the cell consists mostly of water. The dry mass of a single cell can be estimated as 23% of the wet mass, amounting to 0.2 pg. About half of the dry mass of a bacterial cell consists of carbon, and also about half of it can be attributed to proteins. Therefore, a typical fully grown 1-liter culture of ''
Escherichia coli
''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
'' (at an optical density of 1.0, corresponding to c. 10
9 cells/ml) yields about 1 g wet cell mass. Small size is extremely important because it allows for a large
surface area-to-volume ratio which allows for rapid uptake and intracellular distribution of nutrients and excretion of wastes. At low surface area-to-volume ratios the diffusion of nutrients and waste products across the bacterial cell membrane limits the rate at which microbial metabolism can occur, making the cell less evolutionarily fit. The reason for the existence of large cells is unknown, although it is speculated that the increased cell volume is used primarily for storage of excess nutrients.
Comparison of a typical bacterial cell and a typical human cell (assuming both cells are spheres) :
Cell wall

The
cell envelope
The cell envelope comprises the inner cell membrane and the cell wall of a bacterium. In Gram-negative bacteria an bacterial outer membrane, outer membrane is also included. This envelope is not present in the Mollicutes where the cell wall is abse ...
is composed of the
cell membrane
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
and the
cell wall
A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...
. As in other organisms, the
bacterial cell wall provides structural integrity to the cell. In
prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s, the primary function of the cell wall is to protect the cell from internal
turgor pressure caused by the much higher concentrations of proteins and other molecules inside the cell compared to its external environment. The bacterial cell wall differs from that of all other organisms by the presence of
peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
which is located immediately outside of the cell membrane. Peptidoglycan is made up of a polysaccharide backbone consisting of alternating
N-Acetylmuramic acid (NAM) and
N-acetylglucosamine (NAG) residues in equal amounts.
Peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
is responsible for the rigidity of the bacterial cell wall, and for the determination of cell shape. It is relatively porous and is not considered to be a permeability barrier for small substrates. While all bacterial cell walls (with a few exceptions such as extracellular
parasite
Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
s such as ''
Mycoplasma'') contain peptidoglycan, not all cell walls have the same overall structures. Since the cell wall is required for bacterial survival, but is absent in some
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s, several
antibiotics
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...
(notably the
penicillin
Penicillins (P, PCN or PEN) are a group of beta-lactam antibiotic, β-lactam antibiotics originally obtained from ''Penicillium'' Mold (fungus), moulds, principally ''Penicillium chrysogenum, P. chrysogenum'' and ''Penicillium rubens, P. ru ...
s and
cephalosporin
The cephalosporins (sg. ) are a class of β-lactam antibiotics originally derived from the fungus '' Acremonium'', which was previously known as ''Cephalosporium''.
Together with cephamycins, they constitute a subgroup of β-lactam antibio ...
s) stop bacterial infections by interfering with cell wall synthesis, while having no effects on
human cells which have no cell wall, only a cell membrane. There are two main types of bacterial cell walls, those of
Gram-positive bacteria
In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.
The Gram stain ...
and those of
Gram-negative bacteria
Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...
, which are differentiated by their
Gram staining characteristics. For both these types of bacteria, particles of approximately 2 nm can pass through the peptidoglycan. If the bacterial cell wall is entirely removed, it is called a
protoplast while if it's partially removed, it is called a
spheroplast.
Beta-lactam antibiotics such as penicillin inhibit the formation of peptidoglycan cross-links in the bacterial cell wall. The enzyme
lysozyme
Lysozyme (, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside hydrolase ...
, found in human tears, also digests the cell wall of bacteria and is the body's main defense against eye infections.
Gram-positive cell wall
Gram-positive cell walls are thick and the
peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
(also known as ''murein'') layer constitutes almost 95% of the cell wall in some Gram-positive bacteria and as little as 5-10% of the cell wall in Gram-negative bacteria. The peptidoglycan layer takes up the
crystal violet dye and stains purple in the
Gram stain. Bacteria within the ''
Deinococcota'' group may also exhibit Gram-positive staining but contain some cell wall structures typical of Gram-negative bacteria.
The cell wall of some Gram-positive bacteria can be completely dissolved by
lysozyme
Lysozyme (, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside hydrolase ...
s which attack the bonds between N-acetylmuramic acid and N-acetylglucosamine. In other Gram-positive bacteria, such as ''
Staphylococcus aureus
''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often posi ...
'', the walls are resistant to the action of lysozymes. They have O-acetyl groups on carbon-6 of some muramic acid residues.
The matrix substances in the walls of Gram-positive bacteria may be polysaccharides or
teichoic acids. The latter are very widespread, but have been found only in Gram-positive bacteria. There are two main types of teichoic acid: ribitol teichoic acids and glycerol teichoic acids. The latter one is more widespread. These acids are polymers of
ribitol phosphate and
glycerol phosphate, respectively, and only located on the surface of many Gram-positive bacteria. However, the exact function of teichoic acid is debated and not fully understood. Some are lipid-linked to form
lipoteichoic acids. Because lipoteichoic acids are covalently linked to lipids within the
cytoplasmic membrane they are responsible for linking and anchoring the
peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
to the
cytoplasmic membrane. Lipotechoic acid is a major component of the gram-positive cell wall. One of its purposes is providing an antigenic function. The lipid element is to be found in the membrane where its adhesive properties assist in its anchoring to the membrane. Teichoic acids give the gram-positive cell wall an overall negative charge due to the presence of
phosphodiester bonds between teichoic acid monomers.
Outside the cell wall, many gram-positive bacteria have an
S-layer of "tiled" proteins. The S-layer assists attachment and biofilm formation. Outside the S-layer, there is often a
capsule of polysaccharides. The capsule helps the bacterium evade host
phagocytosis
Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...
. In laboratory culture, the S-layer and capsule are often lost by reductive evolution (the loss of a trait in absence of positive selection).
Gram-negative cell wall
Gram-negative cell walls are much thinner than the Gram-positive cell walls, and they contain a second plasma membrane superficial to their thin
peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
layer, in turn adjacent to the
cytoplasmic membrane. Gram-negative bacteria stain as pink in the
Gram stain. The chemical structure of the
outer membrane's lipopolysaccharide is often unique to specific bacterial sub-species and is responsible for many of the
antigenic properties of these strains.
In addition to the peptidoglycan layer the Gram-negative cell wall also contains an additional
outer membrane composed of
phospholipid
Phospholipids 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 an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
s and
lipopolysaccharides which face into the external environment. The highly charged nature of
lipopolysaccharides confer an overall negative charge to the Gram -negative cell wall. The chemical structure of the outer membrane lipopolysaccharides is often unique to specific bacterial strains, and is responsible for many of their
antigenic properties.
As a
phospholipid bilayer, the lipid portion of the outer membrane is largely impermeable to all charged molecules. However, channels called
porins are present in the outer membrane that allow for
passive transport of many
ions,
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...
s and
amino acids across the outer membrane. These molecules are therefore present in the
periplasm, the region between the plasma membrane and outer membrane. The
periplasm contains the peptidoglycan layer and many proteins responsible for substrate binding or
hydrolysis and reception of extracellular signals. The periplasm is thought to exist as a gel-like state rather than a liquid due to the high concentration of proteins and peptidoglycan found within it. Because of its location between the cytoplasmic and outer membranes, signals received and substrates bound are available to be transported across the
cytoplasmic membrane using transport and signaling proteins imbedded there.
Many uncultivated Gram-negative bacteria also have an
S-layer and a
capsule. These structures are often lost during laboratory cultivation.
Plasma membrane
The
plasma membrane or bacterial cytoplasmic membrane is composed of a
phospholipid bilayer and thus has all of the general functions of a
cell membrane
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
such as acting as a permeability barrier for most molecules and serving as the location for the transport of molecules into the cell. In addition to these functions,
prokaryotic membranes also function in energy conservation as the location about which a
proton motive force is generated. Unlike
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s, bacterial membranes (with some exceptions e.g. ''
Mycoplasma'' and
methanotrophs) generally do not contain
sterol
A sterol is any organic compound with a Skeletal formula, skeleton closely related to Cholestanol, cholestan-3-ol. The simplest sterol is gonan-3-ol, which has a formula of , and is derived from that of gonane by replacement of a hydrogen atom on ...
s. However, many microbes do contain structurally related compounds called
hopanoids which likely fulfill the same function. Unlike
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s,
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
can have a wide variety of
fatty acid
In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...
s within their membranes. Along with typical saturated and unsaturated
fatty acid
In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...
s, bacteria can contain fatty acids with additional
methyl,
hydroxy or even cyclic groups. The relative proportions of these fatty acids can be modulated by the bacterium to maintain the optimum fluidity of the membrane (e.g. following temperature change).
Gram-negative and mycobacteria have an inner and outer bacteria membrane. As a
phospholipid bilayer, the lipid portion of the
bacterial outer membrane is impermeable to charged molecules. However, channels called
porins are present in the outer membrane that allow for
passive transport of many
ions,
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...
s and
amino acids across the outer membrane. These molecules are therefore present in the
periplasm, the region between the cytoplasmic and outer membranes. The
periplasm contains the peptidoglycan layer and many proteins responsible for substrate binding or
hydrolysis and reception of extracellular signals. The periplasm is thought to exist in a gel-like state rather than a liquid due to the high concentration of proteins and
peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
found within it. Because of its location between the cytoplasmic and outer membranes, signals received and substrates bound are available to be transported across the
cytoplasmic membrane using transport and signaling proteins imbedded there.
Extracellular (external) structures
Fimbriae and pili
Fimbriae (sometimes called "
attachment pili") are protein tubes that extend out from the outer membrane in many members of the ''
Pseudomonadota''. They are generally short in length and present in high numbers about the entire bacterial cell surface. Fimbriae usually function to facilitate the attachment of a
bacterium to a surface (e.g. to form a
biofilm
A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...
) or to other cells (e.g. animal cells during
pathogenesis). A few organisms (e.g. ''
Myxococcus'') use fimbriae for
motility
Motility is the ability of an organism to move independently using metabolism, metabolic energy. This biological concept encompasses movement at various levels, from whole organisms to cells and subcellular components.
Motility is observed in ...
to facilitate the assembly of multicellular structures such as
fruiting bodies.
Pili are similar in structure to fimbriae but are much longer and present on the bacterial cell in low numbers.
Pili are involved in the process of
bacterial conjugation where they are called
conjugation pili or "
sex pili".
Type IV pili (non-sex pili) also aid bacteria in gripping surfaces.
S-layers
An
S-layer (surface layer) is a cell surface protein layer found in many different
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
and in some
archaea
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
, where it serves as the cell wall. All
S-layers are made up of a two-dimensional array of proteins and have a crystalline appearance, the symmetry of which differs between species. The exact function of
S-layers is unknown, but it has been suggested that they act as a partial permeability barrier for large substrates. For example, an
S-layer could conceivably keep extracellular proteins near the cell membrane by preventing their diffusion away from the cell. In some pathogenic species, an
S-layer may help to facilitate survival within the host by conferring protection against host defence mechanisms.
Glycocalyx
Many bacteria secrete extracellular polymers outside of their cell walls called
glycocalyx. These polymers are usually composed of
polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...
s and sometimes
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
.
Capsules are relatively impermeable structures that cannot be stained with dyes such as
India ink. They are structures that help protect bacteria from
phagocytosis
Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...
and
desiccation.
Slime layer is involved in attachment of bacteria to other cells or inanimate surfaces to form
biofilm
A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...
s. Slime layers can also be used as a food reserve for the cell.
Flagella
Perhaps the most recognizable extracellular bacterial cell structures are
flagella. Flagella are whip-like structures protruding from the bacterial cell wall and are responsible for
bacterial motility (movement). The arrangement of flagella about the bacterial cell is unique to the species observed. Common forms include:
*
Monotrichous – Single flagellum
*
Lophotrichous – A tuft of flagella found at one of the cell poles
*
Amphitrichous – Single flagellum found at each of two opposite poles
*
Peritrichous – Multiple flagella found at several locations about the cell
The
bacterial flagellum consists of three basic components: a whip-like filament, a motor complex, and a hook that connects them. The filament is approximately 20 nm in diameter and consists of several protofilaments, each made up of thousands of
flagellin subunits. The bundle is held together by a cap and may or may not be encapsulated. The motor complex consists of a series of rings anchoring the flagellum in the inner and outer membranes, followed by a proton-driven
motor that drives rotational movement in the filament.
Intracellular (internal) structures

In comparison to
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s, the intracellular features of the bacterial cell are extremely simple. Bacteria do not contain
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...
s in the same sense as
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s. Instead, the
chromosome and perhaps
ribosomes are the only easily observable intracellular structures found in all
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
. There do exist, however, specialized groups of bacteria that contain more complex intracellular structures, some of which are discussed below.
The bacterial DNA and plasmids
Unlike
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s, the bacterial
DNA is not enclosed inside of a membrane-bound
nucleus but instead resides inside the
cytoplasm
The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
. The processes concerning the transfer of genetic information —
translation,
transcription, and
DNA replication — therefore all occur within the same compartment and can interact with other cytoplasmic structures, most notably
ribosomes. Bacterial DNA can be located in two places:
* Bacterial chromosome, located in the irregularly shaped region known as the
nucleoid
*
Extrachromosomal DNA, located outside of the nucleoid region as circular or linear
plasmids
The bacterial DNA is not packaged using
histones to form
chromatin as in
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s but instead exists as a highly compact
supercoiled structure, the precise nature of which remains unclear. Most bacterial chromosomes are
circular, although some examples of linear chromosomes exist (e.g. ''
Borrelia burgdorferi''). Usually, a single bacterial chromosome is present, although some species with multiple chromosomes have been described.
Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmids that often encode advantageous traits but are not essential to their bacterial host. Plasmids can be easily gained or lost by a bacterium and can be transferred between bacteria as a form of
horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
.
Ribosomes and other multiprotein complexes
In most
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
the most numerous intracellular structure is the
ribosome, the site of
protein synthesis in all living organisms. All
prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s have 70S (where S=
Svedberg
In chemistry, a Svedberg unit or svedberg (symbol S, sometimes Sv) is a non- SI metric unit for sedimentation coefficients. The Svedberg unit offers a measure of a particle's size indirectly based on its sedimentation rate under acceleration ...
units) ribosomes while
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s contain larger 80S
ribosomes in their
cytosol. The 70S
ribosome is made up of a 50S and 30S subunits. The 50S subunit contains the 23S and 5S
rRNA while the 30S subunit contains the 16S
rRNA. These
rRNA molecules differ in size in
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s and are complexed with a large number of ribosomal proteins, the number and type of which can vary slightly between organisms. While the
ribosome is the most commonly observed intracellular multiprotein complex in
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
other large complexes do occur and can sometimes be seen using
microscopy.
Intracellular membranes
While not typical of all
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
, some microbes contain intracellular membranes in addition to (or as extensions of) their cytoplasmic membranes. An early idea was that bacteria might contain membrane folds termed
mesosomes, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for
electron microscopy. Examples of
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
containing intracellular membranes are
phototrophs (ex.:
thylakoid),
nitrifying bacteria (ex.:
anammoxosome) and
methane-oxidising bacteria such as the
methylococcaceae. Intracellular membranes are also found in
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
belonging to the poorly studied
Planctomycetota group, although these membranes more closely resemble organellar membranes in
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s and are currently of unknown function.
Chromatophores are intracellular membranes found in
phototrophic bacteria. Used primarily for photosynthesis, they contain
bacteriochlorophyll pigments and carotenoids.
Cytoskeleton
The
prokaryotic cytoskeleton is the collective name for all structural
filaments in
prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s. It was once thought that prokaryotic cells did not possess
cytoskeletons, but advances in imaging technology and structure determination have shown the presence of filaments in these cells.
Homologues for all major cytoskeletal proteins in
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s have been found in prokaryotes. Cytoskeletal elements play essential roles in
cell division, protection, shape determination, and polarity determination in various prokaryotes.
Nutrient storage structures
Most
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
do not live in environments that contain large amounts of nutrients at all times. To accommodate these transient levels of nutrients,
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
contain several different methods of nutrient storage in times of plenty for use in times of want. For example, many
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
store excess carbon in the form of
polyhydroxyalkanoates or
glycogen. Some microbes store soluble nutrients such as
nitrate
Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
in
vacuole
A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...
s. Sulfur is most often stored as elemental (S
0) granules which can be deposited either intra- or extracellularly. Sulfur granules are especially common in
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
that use
hydrogen sulfide as an electron source. Most of the above-mentioned examples can be viewed using a
microscope
A microscope () is a laboratory equipment, laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic ...
and are surrounded by a thin nonunit membrane to separate them from the
cytoplasm
The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
.
Inclusions
Inclusions are considered to be nonliving components of the cell that do not possess metabolic activity and are not bounded by membranes. The most common inclusions are glycogen, lipid droplets, crystals, and pigments.
Volutin granules are cytoplasmic inclusions of complexed inorganic polyphosphate. These granules are called
metachromatic granules due to their displaying the metachromatic effect; they appear red or blue when stained with the blue dyes methylene blue or toluidine blue.
Gas vacuoles
Gas vacuoles are membrane-bound, spindle-shaped
vesicles, found in some
plankton
Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
ic bacteria and ''
Cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
'', that provides
buoyancy to these cells by decreasing their overall cell
density. Positive buoyancy is needed to keep the cells in the upper reaches of the water column, so that they can continue to perform
photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. They are made up of a shell of protein that has a highly
hydrophobic inner surface, making it impermeable to water (and stopping water vapour from condensing inside) but permeable to most
gases. Because the gas vesicle is a hollow cylinder, it is liable to collapse when the surrounding
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
increases. Natural selection has fine tuned the structure of the gas vesicle to maximise its resistance to
buckling, including an external strengthening protein, GvpC, rather like the green thread in a braided hosepipe. There is a simple relationship between the diameter of the gas vesicle and pressure at which it will collapse – the wider the gas vesicle the weaker it becomes. However, wider gas vesicles are more efficient, providing more buoyancy per unit of protein than narrow gas vesicles. Different species produce gas vesicle of different diameter, allowing them to colonise different depths of the water column (fast growing, highly competitive species with wide gas vesicles in the top most layers; slow growing, dark-adapted, species with strong narrow gas vesicles in the deeper layers). The diameter of the gas vesicle will also help determine which species survive in different bodies of water. Deep lakes that experience winter mixing expose the cells to the hydrostatic pressure generated by the full water column. This will select for species with narrower, stronger gas vesicles.
The cell achieves its height in the water column by synthesising gas vesicles. As the cell rises up, it is able to increase its
carbohydrate
A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...
load through increased photosynthesis. Too high and the cell will suffer photobleaching and possible death, however, the carbohydrate produced during photosynthesis increases the cell's density, causing it to sink. The daily cycle of carbohydrate build-up from photosynthesis and carbohydrate
catabolism during dark hours is enough to fine-tune the cell's position in the water column, bring it up toward the surface when its carbohydrate levels are low and it needs to photosynthesis, and allowing it to sink away from the harmful
UV radiation when the cell's carbohydrate levels have been replenished. An extreme excess of carbohydrate causes a significant change in the internal pressure of the cell, which causes the gas vesicles to buckle and collapse and the cell to sink out.
Microcompartments
Bacterial microcompartments are widespread, organelle-like structures that are made of a protein shell that surrounds and encloses various enzymes. provide a further level of organization; they are compartments within bacteria that are surrounded by polyhedral protein shells, rather than by lipid membranes. These "polyhedral organelles" localize and compartmentalize bacterial metabolism, a function performed by the membrane-bound organelles in eukaryotes.
= Carboxysomes
=
Carboxysomes are bacterial microcompartments found in many
autotrophic bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
such as Cyanobacteria, Knallgasbacteria, Nitroso- and Nitrobacteria. They are proteinaceous structures resembling phage heads in their
morphology and contain the enzymes of carbon dioxide fixation in these organisms (especially ribulose bisphosphate carboxylase/oxygenase, RuBisCO, and carbonic anhydrase). It is thought that the high local concentration of the enzymes along with the fast conversion of bicarbonate to carbon dioxide by carbonic anhydrase allows faster and more efficient carbon dioxide fixation than possible inside the cytoplasm. Similar structures are known to harbor the coenzyme B12-containing
glycerol dehydratase, the key enzyme of glycerol fermentation to 1,3-propanediol, in some Enterobacteriaceae (e. g. Salmonella).
= Magnetosomes
=
Magnetosomes are bacterial microcompartments found in
magnetotactic bacteria that allow them to sense and align themselves along a magnetic field (
magnetotaxis). The ecological role of magnetotaxis is unknown but is thought to be involved in the determination of optimal oxygen concentrations. Magnetosomes are composed of the mineral
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...
or
greigite and are surrounded by a lipid bilayer membrane. The morphology of magnetosomes is species-specific.
Endospores
Perhaps the best known bacterial adaptation to stress is the formation of
endospores. Endospores are bacterial survival structures that are highly resistant to many different types of chemical and environmental stresses and therefore enable the survival of
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
in environments that would be lethal for these cells in their normal vegetative form. It has been proposed that endospore formation has allowed for the survival of some bacteria for hundreds of millions of years (e.g. in salt crystals) although these publications have been questioned. Endospore formation is limited to several genera of gram-positive bacteria such as ''
Bacillus'' and ''
Clostridium''. It differs from reproductive spores in that only one spore is formed per cell resulting in no net gain in cell number upon endospore germination. The location of an endospore within a cell is species-specific and can be used to determine the identity of a bacterium.
Dipicolinic acid is a chemical compound which composes 5% to 15% of the dry weight of bacterial spores and is implicated in being responsible for the heat resistance of endospores. Archaeologists have found viable endospores taken from the intestines of Egyptian mummies as well as from lake sediments in Northern Sweden estimated to be many thousands of years old.
[ ]
References
Further reading
Cell Structure and Organization*
External links
{{Anatomy
Bacteria
Bacteriology