Editing Cell wall (section)

==Prokaryotic cell walls==

===Bacterial cell walls===
<!-- This section is linked from [[Bacteriocin]] -->
[[File:Prokaryote cell.svg|thumb|right|Illustration of a typical [[Gram-positive bacteria|gram-positive bacterium]]. The cell envelope comprises a [[cell membrane|plasma membrane]], seen here in light brown, and a thick [[peptidoglycan]]-containing cell wall (the purple layer). No [[bacterial outer membrane|outer lipid membrane]] is present, as would be the case in [[gram-negative bacteria]]. The red layer, known as the [[bacterial capsule|capsule]], is distinct from the cell envelope.]]

{{Further|Cell envelope|Bacterial cell structure}}
Around the outside of the cell membrane is the bacterial cell wall. Bacterial cell walls are made of [[peptidoglycan]] (also called murein), which is made from [[polysaccharide]] chains cross-linked by unusual [[peptide]]s containing D-[[amino acid]]s.<ref>{{cite journal  | author = van Heijenoort J | title = Formation of the glycan chains in the synthesis of bacterial peptidoglycan | url=http://glycob.oxfordjournals.org/cgi/content/full/11/3/25R | journal = Glycobiology | volume = 11 | issue = 3 | pages = 25R – 36R | year = 2001 | pmid = 11320055 | doi = 10.1093/glycob/11.3.25R  | doi-access = free }}</ref> Bacterial cell walls are different from the cell walls of [[plant]]s and [[fungi]] which are made of [[cellulose]] and [[chitin]], respectively.<ref name=Koch>{{cite journal | vauthors = Koch AL | title = Bacterial wall as target for attack: past, present, and future research | journal = Clinical Microbiology Reviews | volume = 16 | issue = 4 | pages = 673–87 | date = October 2003 | pmid = 14557293 | pmc = 207114 | doi = 10.1128/CMR.16.4.673-687.2003 }}</ref> The cell wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, although [[L-form bacteria]] can be produced in the laboratory that lack a cell wall.<ref name=Joseleau>{{cite journal | vauthors = Joseleau-Petit D, Liébart JC, Ayala JA, D'Ari R | title = Unstable Escherichia coli L forms revisited: growth requires peptidoglycan synthesis | journal = Journal of Bacteriology | volume = 189 | issue = 18 | pages = 6512–20 | date = September 2007 | pmid = 17586646 | pmc = 2045188 | doi = 10.1128/JB.00273-07 }}</ref> The antibiotic [[penicillin]] is able to kill bacteria by preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken and lyse.<ref name=Koch/> The [[lysozyme]] enzyme can also damage bacterial cell walls.

There are broadly speaking two different types of cell wall in bacteria, called [[gram-positive]] and [[gram-negative]]. The names originate from the reaction of cells to the [[Gram stain]], a test long-employed for the classification of bacterial species.<ref name=Gram>{{cite journal | last = Gram | first =  HC | author-link = Hans Christian Gram | year = 1884 | title = Über die isolierte Färbung der Schizomyceten in Schnitt- und Trockenpräparaten | journal = Fortschr. Med. | volume = 2 | pages = 185&ndash;189 }}</ref>

Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and [[teichoic acid]]s. 

Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing [[lipopolysaccharide]]s and [[lipoprotein]]s. Most bacteria have the gram-negative cell wall and only the [[Bacillota]] and [[Actinomycetota]] (previously known as the low G+C and high G+C gram-positive bacteria, respectively) have the alternative gram-positive arrangement.<ref>{{cite journal | vauthors = Hugenholtz P | title = Exploring prokaryotic diversity in the genomic era | journal = Genome Biology | volume = 3 | issue = 2 | pages = REVIEWS0003 | year = 2002 | pmid = 11864374 | pmc = 139013 | doi = 10.1186/gb-2002-3-2-reviews0003 | doi-access = free }}</ref> 

These differences in structure produce differences in antibiotic susceptibility. The [[beta-lactam antibiotics]] (e.g. [[penicillin]], [[cephalosporin]]) only work against gram-negative pathogens, such as ''[[Haemophilus influenzae]]'' or ''[[Pseudomonas aeruginosa]]''. The [[glycopeptide antibiotic]]s (e.g. [[vancomycin]], [[teicoplanin]], [[telavancin]]) only work against gram-positive pathogens such as ''[[Staphylococcus aureus]]'' <ref>{{cite journal  |vauthors=Walsh F, Amyes S | title = Microbiology and drug resistance mechanisms of fully resistant pathogens. | journal = Curr Opin Microbiol | volume = 7 | issue = 5 | pages = 439–44 | year = 2004 | pmid = 15451497 | doi = 10.1016/j.mib.2004.08.007  | url = http://mural.maynoothuniversity.ie/13551/1/FW-Microbiology-2004.pdf }}</ref>

===Archaeal cell walls===
Although not truly unique, the cell walls of [[Archaea]] are unusual. Whereas [[peptidoglycan]] is a standard component of all bacterial cell walls, all archaeal cell walls lack [[peptidoglycan]],<ref name="White 1995">{{cite book | last = White | first = David | name-list-style = vanc | date = 1995 | title = The Physiology and Biochemistry of Prokaryotes | pages = 6, 12–21 | location = Oxford | publisher = Oxford University Press | isbn = 978-0-19-508439-9 }}</ref> though some [[methanogen]]s have a cell wall made of a similar polymer called [[pseudopeptidoglycan]].<ref name="Howland 2000" /> There are four types of cell wall currently known among the Archaea.

One type of archaeal cell wall is that composed of [[pseudopeptidoglycan]] (also called [[pseudomurein]]). This type of wall is found in some [[methanogen]]s, such as ''[[Methanobacterium]]'' and ''[[Methanothermus]]''.<ref name="Brock 1994">{{cite book | vauthors = Brock TD, Madigan MT, Martinko JM, Parker J | author-link1 = Thomas D. Brock | date = 1994 | title = Biology of Microorganisms | edition = 7th | pages = 818–819, 824 | location = Englewood Cliffs, NJ | publisher = Prentice Hall | isbn = 978-0-13-042169-2 }}</ref> While the overall structure of archaeal ''pseudo''peptidoglycan superficially resembles that of bacterial peptidoglycan, there are a number of significant chemical differences. Like the peptidoglycan found in bacterial cell walls, pseudopeptidoglycan consists of [[polymer]] chains of [[glycan]] cross-linked by short [[peptide]] connections. However, unlike peptidoglycan, the sugar [[N-Acetylmuramic acid|N-acetylmuramic acid]] is replaced by [[N-Acetyltalosaminuronic acid|N-acetyltalosaminuronic acid]],<ref name="White 1995" /> and the two sugars are bonded with a ''β'',1-3 glycosidic linkage instead of ''β'',1-4. Additionally, the cross-linking peptides are [[L-amino acid]]s rather than D-amino acids as they are in bacteria.<ref name="Brock 1994" />

A second type of archaeal cell wall is found in ''[[Methanosarcina]]'' and ''[[Halococcus]]''. This type of cell wall is composed entirely of a thick layer of [[polysaccharide]]s, which may be [[sulfate]]d in the case of ''Halococcus''.<ref name="Brock 1994" /> Structure in this type of wall is complex and not fully investigated.

A third type of wall among the [[Archaea]] consists of [[glycoprotein]], and occurs in the [[hyperthermophile]]s, ''[[Halobacterium]]'', and some [[methanogen]]s. In ''Halobacterium'', the [[protein]]s in the wall have a high content of [[acid]]ic [[amino acid]]s, giving the wall an overall negative charge. The result is an unstable structure that is stabilized by the presence of large quantities of positive [[sodium]] [[ion]]s that [[Neutralization (chemistry)|neutralize]] the charge.<ref name="Brock 1994" /> Consequently, ''Halobacterium'' thrives only under conditions with high [[salinity]].

In other Archaea, such as ''[[Methanomicrobium]]'' and ''[[Desulfurococcus]]'', the wall may be composed only of surface-layer [[protein]]s,<ref name="Howland 2000"/> known as an ''[[S-layer]]''. S-layers are common in bacteria, where they serve as either the sole cell-wall component or an outer layer in conjunction with [[polysaccharides]]. Most Archaea are Gram-negative, though at least one Gram-positive member is known.<ref name="Howland 2000" />