Bacterial cell structure
4209093
222681640
2008-06-30T16:20:39Z
TimVickers
1635280
/* The Gram negative cell wall */ reword
[[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 [[bacteria]] and are not found among [[archaea]] or [[eukaryotes]]. 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 principles that have been subsequently applied to other organisms.
==Cell morphology==
[[Image:Bacterial morphology diagram.svg|right|thumb|350px|Bacteria come in a wide variety of shapes]]
Perhaps the most elemental structural property of [[bacteria]] is [[morphology (biology)|cell morphology]] (shape). Typical examples include:
* [[coccus]] (spherical)
* bacillus (rod-like) <!-- Do not link to the genus Bacillus. -->
* [[spirillum]] (spiral)
* [[Filamentation|filamentous]]
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]] generally form distinctive cell morphologies when examined by [[microscopy|light microscopy]] and distinct colony morphologies when grown on [[Petri dishes|Petri plates]]. These are often the first characteristics observed by a [[microbiologist]] to determine the identity of an unknown [[bacterial culture]].
==The importance of cell size==
Perhaps the most obvious structural characteristic of [[bacteria]] is (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 μm<sup>3</sup>. 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. 10<sup>9</sup> cells/ml) yields ca. 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.
==The bacterial cell wall==
{{mainarticle|cell envelope}}
[[Image:Muréine.JPG|right|thumb|180px|The structure of [[peptidoglycan]].]]
As in other organisms, the bacterial [[cell wall]] provides structural integrity to the cell. In [[prokaryotes]], 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]] (poly-''N''-acetylglucosamine and ''N''-acetylmuramic acid), which is located immediately outside of the [[cytoplasmic membrane]]. [[Peptidoglycan]] 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 permiability 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 staining]] characteristics. For both Gram-positive and Gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan.<ref>{{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]</ref>
===The Gram positive cell wall===
The [[Gram positive]] cell wall is characterized by the presence of a very thick [[peptidoglycan]] layer, which is responsible for the retention of the crystal violet dyes during the [[Gram staining]] procedure. 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 positive]] organisms). Bacteria within the [[Deinococcus-Thermus]] group may also exhibit [[Gram positive]] staining behaviour but contain some cell wall structures typical of [[Gram negative]] organisms. Imbedded in the Gram positive cell wall are polyalcohols called [[teichoic acid]]s, some of which are lipid-linked to form [[teichoic acid|lipoteichoic acids]]. Because [[teichoic acid|lipoteichoic acids]] are covalently linked to lipids within the [[cytoplasmic membrane]] they are responsible for linking the [[peptidoglycan]] to the [[cytoplasmic membrane]]. [[Teichoic acid]]s give the [[Gram positive]] cell wall an overall negative charge due to the presence of [[phosphodiester bonds]] between [[teichoic acid]] monomers.
===The Gram negative cell wall===
Unlike the Gram positive cell wall, the Gram negative cell wall contains a thin [[peptidoglycan]] layer 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 [[peptidoglycan]] layer, the Gram negative cell wall also contains an additional outer membrane composed by [[phospholipid]]s and [[lipopolysaccharide]]s which face into the external environment. As the [[lipopolysaccharide]]s are highly-charged, the Gram negative cell wall has an overall negative charge. The chemical structure of the outer membrane [[lipopolysaccharide]]s is often unique to specific bacterial strains (i.e. sub-species) and is responsible for many of the [[antigen]]ic properties of these strains.
==The bacterial cytoplasmic membrane==
The bacterial cytoplasmic membrane is composed of a [[phospholipid bilayer]] and thus has all of the general functions of a [[cell membrane]] 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 [[eukaryotes]], bacterial membranes (with some exceptions e.g. ''[[Mycoplasma]]'' and [[Methanotrophic|methanotrophs]]) generally do not contain [[sterols]]. However, many microbes do contain structurally related compounds called [[hopanoid]]s which likely fulfill the same function. Unlike [[eukaryotes]], [[bacteria]] can have a wide variety of [[fatty acid]]s within their membranes. Along with typical saturated and unsaturated [[fatty acid]]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).
As a [[phospholipid bilayer]], the lipid portion of the outer membrane is impermeable to charged molecules. However, channels called [[Porin (protein)|porin]]s are present in the outer membrane that allow for [[passive transport]] of many [[ion]]s, [[sugar]]s and [[amino acid]]s 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 it 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 signalling proteins imbedded there.
==Other bacterial surface structures==
===Fimbrae and Pili===
''Main article:'' [[Pilus]]
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 [[bacterium]] to 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 body|fruiting bodies]]. [[Pilus|Pili]] are similar in structure to fimbrae but are much longer and present on the bacterial cell in low numbers. [[Pilus|Pili]] are involved in the process of [[bacterial conjugation]]. Non-sex pili also aid bacteria in gripping surfaces.
===S-layers===
''Main article:'' [[S-layer]]
An [[S-layer]] is a cell surface protein layer found in many different [[bacteria]] and in some [[archaea]] where it serves as the cell wall. All [[S-layer]]s 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-layer]]s 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.
===Capsules and Slime Layers===
''Main article:'' [[Slime layer]]
Many [[bacteria]] secrete extracellular polymers outside of their cell walls. These polymers are usually composed of [[polysaccharide]]s and sometimes [[protein]]. Capsules are relatively impermeable structures that cannot be stained with dyes such as [[India ink]]. They are structures that help protect [[bacteria]] from [[phagocytosis]] and [[desiccation]]. [[Slime layer]]s are somewhat looser, fibrous structures generally involved in attachment of [[bacteria]] to other cells or inanimate surfaces to form [[biofilm]]s. 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).
[[image:flagella.png|thumb|200px|right|
A-[[Monotrichous;]]
B-[[Lophotrichous;]]
C-[[Amphitrichous;]]
D-[[Peritrichous;]]]]
===Flagella===
''Main article:'' [[Flagellum]]
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]] (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
[[Flagella]] are complex structures that are composed of many different proteins. These include [[flagellin]], which makes up the whip-like tube and a [[protein]] complex that spans the cell wall and cell membrane to form a motor that causes the [[flagellum]] to rotate. This rotation is normally driven by [[proton motive force]] and are found in the body of the cell.
==Intracellular bacterial cell structures==
[[Image:Average prokaryote cell- en.svg|thumb|320px|left|[[Prokaryote]] cell structure]]
In comparison to [[eukaryotes]], the intracellular features of the bacterial cell are extremely simplistic. Bacteria do not contain [[organelle]]s in the same sense as [[eukaryote]]s. Instead, the [[chromosome]] and perhaps [[ribosomes]] are 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===
''Main article:'' [[Plasmid]]
Unlike [[eukaryotes]], the bacterial [[chromosome]] is not enclosed inside of a membrane-bound [[cell nucleus|nucleus]] but instead resides inside the bacterial [[cytoplasm]]. This means that the transfer of cellular information through the processes of [[translation]], [[Transcription (genetics)|transcription]] and [[DNA replication]] all occur within the same compartment and can interact with other cytoplasmic structures, most notably [[ribosome]]s. The bacterial chromosome is not packaged using [[histones]] to form [[chromatin]] as in [[eukaryote]]s but instead exists as a highly compact [[supercoil]]ed 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 [[plasmid]]s that often encode for traits that are advantageous but not essential to their bacterial host. [[Plasmid]]s 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===
''Main article:'' [[Ribosome]]
In most [[bacteria]] the most numerous intracellular structure is the [[ribosome]], the site of [[protein biosynthesis|protein synthesis]] in all living organisms. All [[prokaryotes]] have 70S (where S=[[Svedberg]] units) ribosomes while [[eukaryotes]] contain larger 80S [[ribosome]]s 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 [[eukaryotes]] 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]] other large complexes do occur and can sometimes be seen using [[microscopy]].
===Intracellular membranes===
While not typical of all [[bacteria]] 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 [[mesosome]]s, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for [[electron microscopy]].<ref>{{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}}</ref> Examples of [[bacteria]] containing intracellular membranes are [[phototroph]]s, [[nitrifying bacteria]] and [[methane]]-[[oxidise|oxidising]] bacteria. Intracellular membranes are also found in [[bacteria]] belonging to the poorly studied [[Planctomycetes]] group, although these membranes more closely resemble organellar membranes in [[eukaryotes]] and are currently of unknown function.<ref>{{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}}</ref>
===Cytoskeleton===
{{main|Prokaryotic cytoskeleton}}
The prokaryotic cytoskeleton is the collective name for all structural [[protein filament | filaments]] in [[prokaryotes]]. It was once thought that prokaryotic cells did not possess [[cytoskeleton | cytoskeletons]], but recent advances in visualization technology and structure determination have shown that filaments indeed exist in these cells.<ref name="Gitai2005">{{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}}</ref> In fact, [[homologues]] for all major cytoskeletal proteins in [[eukaryotes]] have been found in prokaryotes. Cytoskeletal elements play essential roles in [[cell division]], protection, shape determination, and polarity determination in various prokaryotes.<ref>{{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}}</ref>
===Nutrient storage structures===
Most [[bacteria]]l habitats do not live in environments that contain large amounts of essential nutrients at all times. To accommodate these transient levels of nutrients [[bacteria]] contain several different methods of nutrient storage in times of plenty for use in times of want. For example, many [[bacteria]] store excess carbon in the form of [[polyhydroxybutyrate|polyhydroxyalkanoates]] or [[glycogen]]. Some microbes store soluble nutrients such as [[nitrate]] in [[vacuole]]s. Sulfur is most often stored as elemental (S<sup>0</sup>) granules which can be deposited either intra- or extracellularly. Sulfur granules are especially common in [[bacteria]] that use [[hydrogen sulfide]] as an electron source. Most of the above mentioned examples can be viewed using a [[microscope]] and are surrounded a thin nonunit membrane to separate them from the [[cytoplasm]].
===Gas vesicles===
Gas vesicles are spindle-shaped structures found in some [[plankton]]ic bacteria that provides [[buoyancy]] to 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 [[bacteria]] can 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.
===Carboxysomes===
''Main article:'' [[Carboxysome]]
[[Carboxysome]]s are intracellular structures found in many [[autotrophic]] [[bacteria]] such as Cyanobacteria, Knallgasbacteria, Nitroso- and Nitrobacteria. They are proteinaceous structures resembling phage heads in their [[morphology (biology)|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===
''Main article:'' [[Magnetosome]]
[[Magnetosome]]s are intracellular structures 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 it is hypothesized to be involved in the determination of optimal oxygen concentrations. [[Magnetosome]]s are composed of the mineral [[magnetite]] and are surrounded by a nonunit membrane. The morphology of [[magnetosome]]s is species-specific.
==Endospores==
''Main article:'' [[Endospores]]
Perhaps the most well known bacterial adaptation to stress is the formation of [[endospore]]s. [[Endospore]]s are bacterial survival structures that are highly resistant to many different types of chemical and environmental stresses and therefore enable the survival of [[bacteria]] 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)<ref>[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.]</ref><ref>[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.]</ref> although these publications have been questioned.<ref>[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.]</ref><ref>[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.]</ref> [[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]].
== References ==
<references/>
==Further reading==
* [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.
==External links==
[http://www.blackwellpublishing.com/trun/artwork/Animations/Overview/overview.html Animated guide to bacterial cell structure.]
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