Editing Cell envelope

{{Short description|Protective outer structure of bacterial cells}}
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.<ref>{{DorlandsDict|three/000035855|envelope}}</ref> This envelope is not present in the [[Mollicutes]] where the cell wall is absent.

Bacterial cell envelopes fall into two major categories: a [[Gram-positive]] type which stains purple during [[Gram staining]] and a Gram-negative type which stains pink during Gram staining. Either type may have an enclosing [[bacterial capsule|capsule]] of [[polysaccharides]] for extra protection. As a group these are known as [[polysaccharide encapsulated bacteria]].

==Function==
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 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. This is notably expressed through the classification into Gram-positive and Gram-negative bacteria.

==Types==

===The Gram-positive cell wall===
[[Image:Gram-positive cellwall-schematic.png|thumb|Schematic of typical [[Gram-positive cell wall]] showing arrangement of [[N-Acetylglucosamine]] and [[N-Acetylmuramic acid]]; [[Teichoic acid]]s not shown.]]
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 [[Actinomycetota]] (or high %G+C Gram-positive organisms) and the [[Bacillota]] (or low %G+C Gram-positive organisms). Bacteria within the [[Deinococcota]] group may also exhibit Gram-positive staining behavior 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 [[lipoteichoic acid]]s. Because lipoteichoic acids are covalently linked to lipids within the [[cytoplasmic membrane]] they are responsible for linking the [[peptidoglycan]] to the [[cytoplasmic 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 (microbiology)|capsule]] of polysaccharides. The capsule helps the bacterium evade host [[phagocytosis]]. In laboratory culture, the S-layer and capsule are often lost by reductive evolution (the loss of a trait in absence of positive selection).

===The Gram-negative cell wall===
[[Image:Gram negative cell wall.svg|thumb|350px|Schematic of typical [[Gram-negative cell wall]] showing arrangement of [[N-Acetylglucosamine]] and [[N-Acetylmuramic acid]] and the outer membrane containing [[lipopolysaccharide]].]]
The [[Gram-negative cell wall]] contains a thinner [[peptidoglycan]] layer adjacent to the [[cytoplasmic membrane]] than the Gram-positive wall, 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. The highly charged nature of [[lipopolysaccharide]]s confer an overall negative charge to the Gram -negative cell wall. 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.

As a [[phospholipid bilayer]], the lipid portion of the outer membrane is largely impermeable to all 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 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.

In nature, many uncultivated Gram-negative bacteria also have an [[S-layer]] and a [[Capsule (microbiology)|capsule]]. These structures are often lost during laboratory cultivation.

===Mycobacteria===
The ''[[Mycobacteria]]'' ([[acid-fast]] bacteria) have a cell envelope which is not typical of Gram-positives or Gram-negatives. The mycobacterial cell envelope does not consist of the outer membrane characteristic of Gram-negatives, but has a significant peptidoglycan-arabinogalactan-mycolic acid wall structure which provides an external permeability barrier. Therefore, there is thought to be a distinct 'pseudoperiplasm' compartment between the cytoplasmic membrane and this outer barrier. The nature of this compartment is not well understood.<ref>IC Sutcliffe, DJ Harrington.Lipoproteins of mycobacterium tuberculosis: an abundant and functionally diverse class of cell envelope components. FEMS Microbiology Reviews 28 (2004) 645-759</ref> [[Acid-fast|Acid-fast bacteria]], like ''[[Mycobacteria]]'', are resistant to decolorization by acids during staining procedures. The high [[mycolic acid]] content of ''Mycobacteria'', is responsible for the staining pattern of poor absorption followed by high retention. The most common staining technique used to identify acid-fast bacteria is the [[Ziehl–Neelsen stain]] or acid-fast stain, in which the acid fast bacilli are stained bright red and stand out clearly against a blue background.

===Bacteria lacking a peptidoglycan cell wall===
The obligate intracellular bacteria in the family [[Chlamydiaceae]] are unique in their morphology as they do not contain detectable amounts of peptidoglycan in the cell wall of their infectious forms.<ref>Chopra I, Storey C, Falla TJ, Pearce JH. Antibiotics, peptidoglycan synthesis and genomics: the chlamydial anomaly revisited. Microbiology. 1998 144 ( Pt 10):2673-8.</ref> Instead, the extracellular forms of these Gram-negative bacteria maintain their structural integrity by relying on a layer of disulfide bond cross-linked cysteine-rich proteins, which is located between cytoplasmic membrane and outer membrane in a manner analogous to the peptidoglycan layer in other Gram-negative bacteria.<ref>Hatch TP. Disulfide cross-linked envelope proteins: the functional equivalent of peptidoglycan in chlamydiae? J. Bacteriol. 1996 178:1–5.</ref> In the intracellular forms of the bacterium the disulfide cross linkage is not found, which confers this form more mechanically fragile.

The cell envelopes of the bacterial class of [[mollicute]]s do not have a cell wall.<ref name = "Rottem_2003">{{cite journal | author = Rottem S | title = Interaction of mycoplasmas with host cells | journal = Physiol. Rev. | volume = 83 | issue = 2 | pages = 417–32 |date=April 2003 | pmid = 12663864 | doi = 10.1152/physrev.00030.2002 }}</ref> The main [[pathogenic bacteria]] in this class are [[mycoplasma]] and [[ureaplasma]].<ref name="Rottem_2003"/>

[[L-form bacteria]] are strains bacteria that lack cell walls derived from bacteria that normally possess cell walls.<ref name="pmid19212404">{{cite journal | vauthors = Leaver M, Domínguez-Cuevas P, Coxhead JM, Daniel RA, Errington J | title = Life without a wall or division machine in Bacillus subtilis | journal = Nature | volume = 457 | issue = 7231 | pages = 849–53 |date=February 2009 | pmid = 19212404 | doi = 10.1038/nature07742}}</ref>

==See also==
* [[Viral envelope]]

==References==
{{reflist|2}}

{{Organelles}}
{{Bacteria}}

{{DEFAULTSORT:Cell Envelope}}
[[Category:Cells]]
[[Category:Bacteria]]