- Bacterial cell structure
Bacteria, despite their apparent simplicity, contain a well developed cell structure which is responsible for many of their unique biological properties. Many structural features are unique to bacteriaand are not found among archaeaor eukaryotes. Because of the simplicity of bacteriarelative to larger organisms and the ease with which they can be manipulated experimentally, the cell structure of bacteriahas been well studied, revealing many biochemical principles that have been subsequently applied to other organisms.
Perhaps the most elemental structural property of
bacteriais cell morphology (shape). Typical examples include:
* bacillus (rod-like)
* filamentousCell 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"). Bacteriagenerally form distinctive cell morphologies when examined by light microscopy and distinct colony morphologies when grown on Petri plates. These are often the first characteristics observed by a microbiologistto determine the identity of an unknown bacterial culture.
The importance of cell size
Perhaps the most obvious structural characteristic of
bacteriais (with some exceptions) their small size. For example, " Escherichia coli", an "average" sized bacterium with average cell length of ca. 1 µm has a cell volume of approximately 1 - 2 μm3. This corresponds to a wet mass of ca. 1 pg, assuming that the cell consists mostly of water. The dry mass of a single cell can be estimated as 20 % 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" (at an optical density of 1.0, corresponding to ca. 109 cells/ml) yields ca. 1 g wet cell mass.
Small size is extremely important because it allows for a large
surface area-to-volume ratiowhich 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.
The bacterial cell wall
As in other organisms, the bacterial
cell wallprovides structural integrity to the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor pressurecaused 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(poly-"N"-acetylglucosamine and "N"-acetylmuramic acid), which is located immediately outside of the cytoplasmic membrane. Peptidoglycanis 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 e.g. intracellular parasites such as " Mycoplasma") contain peptidoglycan, not all cell walls have the same overall structures. There are two main types of bacterial cell walls, Gram positive and Gram negative, which are differentiated by their Gram stainingcharacteristics. For both Gram-positive and Gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan. [cite journal | author=Demchick PH and Koch AL | title=The permeability of the wall fabric of Escherichia coli and Bacillus subtilis | journal=Journal of Bacteriology | year=1996 | pages=768–73| volume=178 | issue=3 [http://jb.asm.org/cgi/reprint/178/3/768] ]
The Gram positive cell wall
Gram positivecell wall is characterized by the presence of a very thick peptidoglycanlayer, which is responsible for the retention of the crystal violet dyes during the Gram stainingprocedure. It is found exclusively in organisms belonging to the Actinobacteria(or high %G+C Gram positive organisms) and the Firmicutes(or low %G+C Gram positiveorganisms). Bacteria within the Deinococcus-Thermusgroup may also exhibit Gram positivestaining behaviour but contain some cell wall structures typical of Gram negativeorganisms. Embedded in the Gram positive cell wall are polyalcohols called teichoic acids, some of which are lipid-linked to form lipoteichoic acids. Because lipoteichoic acids are covalently linked to lipids within the cytoplasmic membranethey are responsible for linking the peptidoglycanto the cytoplasmic membrane. Teichoic acids give the Gram positivecell wall an overall negative charge due to the presence of phosphodiester bondsbetween teichoic acidmonomers.
The Gram negative cell wall
Unlike the Gram positive cell wall, the Gram negative cell wall contains a thin
peptidoglycanlayer adjacent to the cytoplasmic membrane, which is responsible for the cell wall's inability to retain the crystal violet stain upon decolourisation with ethanol during Gram staining. In addition to the peptidoglycanlayer, the Gram negative cell wall also contains an additional outer membrane composed by phospholipids and lipopolysaccharides which face into the external environment. As the lipopolysaccharides are highly-charged, the Gram negative cell wall has an overall negative charge. The chemical structure of the outer membrane lipopolysaccharides is often unique to specific bacterial strains (i.e. sub-species) and is responsible for many of the antigenic properties of these strains.
The bacterial cytoplasmic membrane
The bacterial cytoplasmic membrane is composed of a
phospholipid bilayerand thus has all of the general functions of a cell membranesuch 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, prokaryoticmembranes also function in energy conservation as the location about which a proton motive forceis generated. Unlike eukaryotes, bacterial membranes (with some exceptions e.g. " Mycoplasma" and methanotrophs) generally do not contain sterols. However, many microbes do contain structurally related compounds called hopanoids which likely fulfill the same function. Unlike eukaryotes, bacteriacan have a wide variety of fatty acids within their membranes. Along with typical saturated and unsaturated fatty acids, bacteria can contain fatty acids with additional methyl, hydroxyor 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).
phospholipid bilayer, the lipid portion of the outer membrane is impermeable to charged molecules. However, channels called porins are present in the outer membrane that allow for passive transportof many ions, sugars and amino acids across the outer membrane. These molecules are therefore present in the periplasm, the region between the cytoplasmic and outer membranes. The periplasmcontains the peptidoglycan layer and many proteins responsible for substrate binding or hydrolysisand reception of extracellular signals. The periplasm it is thought to exist as a gel-like state rather than a liquid due to the high concentration of proteins and peptidoglycanfound 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 membraneusing transport and signalling proteins imbedded there.
Other bacterial surface structures
Fimbrae and Pili
Fimbrae are protein tubes that extend out from the outer membrane in many members of the
Proteobacteria. They are generally short in length and present in high numbers about the entire bacterial cell surface. Fimbrae usually function to facilitate the attachment of a bacteriumto a surface (e.g. to form a biofilm) or to other cells (e.g. animal cells during pathogenesis)). A few organisms (e.g. " Myxococcus") use fimbrae for motility to facilitate the assembly of multicellular structures such as fruiting bodies. Pili are similar in structure to fimbrae but are much longer and present on the bacterial cell in low numbers. Pili are involved in the process of bacterial conjugation. Non-sex pili also aid bacteria in gripping surfaces.
S-layeris a cell surface protein layer found in many different bacteriaand in some archaeawhere 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-layercould conceivably keep extracellular proteins near the cell membrane by preventing their diffusion away from the cell. In some pathogenic species, an S-layermay help to facilitate survival within the host by conferring protection against host defence mechanisms.
Capsules and Slime Layers
bacteriasecrete extracellular polymers outside of their cell walls. These polymers are usually composed of polysaccharides and sometimes protein. Capsules are relatively impermeable structures that cannot be stained with dyes such as India ink. They are structures that help protect bacteriafrom phagocytosisand desiccation. Slime layers are somewhat looser, fibrous structures generally involved in attachment of bacteriato other cells or inanimate surfaces to form biofilms. Slime layers can also be used as a food reserve for the cell.
*An example of how a bacterial cell uses their slime layer to attach to a surface is in the Streptococcus mutans. Streptococcus mutans attaches to the teeth with a slime layer and forms a sticky film that traps food particles and other bacteria on the teeth (dental plaque). The bacteria then metabolizes the trapped food particles and release acids (thus possibly causing tooth decay).
Monotrichous;B- Lophotrichous;C- Amphitrichous;D- Peritrichous;]
Perhaps the most recognizable extracellular bacterial cell structures are
flagella. Flagellaare whip-like structures protruding from the bacterial cell wall and are responsible for bacterial motility(i.e. movement). The arrangement of flagella about the bacterial cell is unique to the species observed. Common forms include:
Peritrichous- Multiple flagella found at several locations about the cell
Polar- Single flagella found at one or both cell poles
Lophotrichous- A tuft of flagella found at one cell pole Flagellaare complex structures that are composed of many different proteins. These include flagellin, which makes up the whip-like tube and a proteincomplex that spans the cell wall and cell membrane to form a motor that causes the flagellumto rotate. This rotation is normally driven by proton motive forceand are found in the body of the cell.
Intracellular bacterial cell structures
In comparison to
eukaryotes, the intracellular features of the bacterial cell are extremely simplistic. Bacteria do not contain organelles in the same sense as eukaryotes. Instead, the chromosomeand perhaps ribosomesare the only easily observable intracellular structures found in all bacteria. There do exist, however, specialized groups of bacteria that contain more complex intracellular structures, some of which are discussed below.
The bacterial chromosome and plasmids
eukaryotes, the bacterial chromosomeis not enclosed inside of a membrane-bound nucleus but instead resides inside the bacterial cytoplasm. This means that the transfer of cellular information through the processes of translation, transcription and DNA replicationall occur within the same compartment and can interact with other cytoplasmic structures, most notably ribosomes. The bacterial chromosome is not packaged using histonesto form chromatinas in eukaryotes 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"). Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmids that often encode for traits that are advantageous but 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.
Ribosomes and other multiprotein complexes
bacteriathe most numerous intracellular structure is the ribosome, the site of protein synthesis in all living organisms. All prokaryoteshave 70S (where S= Svedbergunits) ribosomes while eukaryotescontain larger 80S ribosomes in their cytosol. The 70S ribosomeis made up of a 50S and 30S subunits. The 50S subunit contains the 23S and 5S rRNAwhile the 30S subunit contains the 16S rRNA. These rRNAmolecules differ in size in eukaryotesand are complexed with a large number of ribosomal proteins, the number and type of which can vary slightly between organisms. While the ribosomeis the most commonly observed intracellular multiprotein complex in bacteriaother large complexes do occur and can sometimes be seen using microscopy.
While not typical of all
bacteriasome 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. [cite journal |author=Ryter A |title=Contribution of new cryomethods to a better knowledge of bacterial anatomy |journal=Ann. Inst. Pasteur Microbiol. |volume=139 |issue=1 |pages=33–44 |year=1988 |pmid=3289587] Examples of bacteriacontaining intracellular membranes are phototrophs, nitrifying bacteriaand methane-oxidising bacteria. Intracellular membranes are also found in bacteriabelonging to the poorly studied Planctomycetesgroup, although these membranes more closely resemble organellar membranes in eukaryotesand are currently of unknown function. [cite journal |author=Fuerst J |title=Intracellular compartmentation in planctomycetes |journal=Annu Rev Microbiol |volume=59 |pages=299–328 |year=2005 |pmid=15910279 |doi=10.1146/annurev.micro.59.030804.121258]
The prokaryotic cytoskeleton is the collective name for all structural filaments in
prokaryotes. It was once thought that prokaryotic cells did not possess cytoskeletons, but recent advances in visualization technology and structure determination have shown that filaments indeed exist in these cells.cite journal |author=Gitai Z |title=The new bacterial cell biology: moving parts and subcellular architecture |journal=Cell |volume=120 |issue=5 |pages=577–86 |year=2005 |pmid=15766522 |doi=10.1016/j.cell.2005.02.026] In fact, homologuesfor all major cytoskeletal proteins in eukaryoteshave been found in prokaryotes. Cytoskeletal elements play essential roles in cell division, protection, shape determination, and polarity determination in various prokaryotes. [cite journal |author=Shih YL, Rothfield L |title=The bacterial cytoskeleton |journal=Microbiol. Mol. Biol. Rev. |volume=70 |issue=3 |pages=729–54 |year=2006 |pmid=16959967 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=16959967 |doi=10.1128/MMBR.00017-06]
Nutrient storage structures
bacterial habitats do not live in environments that contain large amounts of essential nutrients at all times. To accommodate these transient levels of nutrients bacteriacontain several different methods of nutrient storage in times of plenty for use in times of want. For example, many bacteriastore excess carbon in the form of polyhydroxyalkanoates or glycogen. Some microbes store soluble nutrients such as nitratein vacuoles. Sulfur is most often stored as elemental (S0) granules which can be deposited either intra- or extracellularly. Sulfur granules are especially common in bacteriathat use hydrogen sulfideas an electron source. Most of the above mentioned examples can be viewed using a microscopeand are surrounded by a thin nonunit membrane to separate them from the cytoplasm.
Gas vesicles are spindle-shaped structures found in some
planktonic bacteria that provides buoyancyto these cells by decreasing their overall cell density. They are made up of a protein coat that is very impermeable to solvents such as water but permeable to most gases. By adjusting the amount of gas present in their gas vesicles bacteriacan increase or decrease their overall cell density and thereby move up or down within the water column to maintain their position in an environment optimal for growth.
Carboxysome Carboxysomes are intracellular structures found in many autotrophic bacteriasuch 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).
Magnetosome Magnetosomes are intracellular structures found in magnetotactic bacteriathat allow them to sense and align themselves along a magnetic field (magnetotaxis). The ecological role of magnetotaxis is unknown but it is hypothesized to be involved in the determination of optimal oxygen concentrations. Magnetosomes are composed of the mineral magnetiteand are surrounded by a nonunit membrane. The morphology of magnetosomes is species-specific.
Perhaps the most well 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 bacteriain environments that would be lethal for these cells in their normal vegetative form. It has been proposed that endosporeformation has allowed for the survival of some bacteriafor hundreds of millions of years (e.g. in salt crystals) [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=11057666&query_hl=11&itool=pubmed_docsum Vreeland RH, Rosenzweig WD, Powers DW. "Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal." Nature. 2000 Oct 19;407(6806):897-900.] ] [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?itool=abstractplus&db=pubmed&cmd=Retrieve&dopt=abstractplus&list_uids=7538699 Cano RJ, Borucki MK. "Revival and identification of bacterial spores in 25- to 40-million-year-old Dominican amber." Science. 1995 May 19;268(5213):1060-4.] ] although these publications have been questioned. [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=7754393&itool=pubmed_abstractplus Fischman J. "Have 25-million-year-old bacteria returned to life?" Science. 1995 May 19;268(5213):977.] ] [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=11057647&itool=pubmed_AbstractPlus Parkes RJ. "A case of bacterial immortality?" Nature. 2000 Oct 19;407(6806):844-5.] ] Endosporeformation is limited to several genera of Gram-positive bacteriasuch 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 endosporegermination. The location of an endosporewithin a cell is species-specific and can be used to determine the identity of a bacterium.
* [http://www.microbiologytext.com/index.php?module=Book&func=displayarticlesinchapter&chap_id=35 Cell Structure and Organization]
* Madigan, M. T., Martinko, J. M. "Brock Biology of Microorganisms, 11th Ed." (2005) Pearson Prentice Hall, Upper Saddle River, NJ.
[http://www.blackwellpublishing.com/trun/artwork/Animations/Overview/overview.html Animated guide to bacterial cell structure.]
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