Editing Polysaccharide (section)

==Bacterial polysaccharides==
{{More citations needed section|date=February 2021}}
[[Pathogenic bacteria]] commonly produce a [[bacterial capsule]], a thick, mucus-like layer of polysaccharide. The capsule cloaks [[antigen]]ic [[protein]]s on the bacterial surface that would otherwise provoke an immune response and thereby lead to the destruction of the bacteria.  Capsular polysaccharides are water-soluble, commonly acidic, and have [[molecular weight]]s on the order of 100,000 to 2,000,000 [[atomic mass unit|daltons]].  They are linear and consist of regularly repeating subunits of one to six [[monosaccharide]]s.  There is enormous structural diversity; nearly two hundred different polysaccharides are produced by ''[[Escherichia coli|E. coli]]'' alone.  Mixtures of capsular polysaccharides, either [[conjugate vaccine|conjugated]] or native, are used as [[vaccine]]s.<ref>{{Cite journal |last=Seeberger |first=Peter H. |date=2021-04-14 |title=Discovery of Semi- and Fully-Synthetic Carbohydrate Vaccines Against Bacterial Infections Using a Medicinal Chemistry Approach: Focus Review |journal=Chemical Reviews |language=en |volume=121 |issue=7 |pages=3598–3626 |doi=10.1021/acs.chemrev.0c01210 |issn=0009-2665 |pmc=8154330 |pmid=33794090}}</ref>

Bacteria and many other microbes, including [[fungi]] and [[algae]], often secrete polysaccharides to help them adhere to surfaces and to prevent them from drying out.<ref>{{Citation |last1=Misra |first1=Swati |title=Bacterial Polysaccharides: An Overview |date=2014 |work=Polysaccharides |pages=1–24 |editor-last=Ramawat |editor-first=Kishan Gopal |url=https://link.springer.com/10.1007/978-3-319-03751-6_68-1 |access-date=2024-06-01 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-03751-6_68-1 |isbn=978-3-319-03751-6 |last2=Sharma |first2=Varsha |last3=Srivastava |first3=Ashok Kumar |editor2-last=Mérillon |editor2-first=Jean-Michel}}</ref>  Humans have developed some of these polysaccharides into useful products, including [[xanthan gum]], [[dextran]], [[welan gum]], [[gellan gum]], diutan gum and [[pullulan]].

Most of these polysaccharides exhibit useful [[visco-elastic]] properties when dissolved in water at very low levels.<ref>Viscosity of Welan Gum vs. Concentration in Water. {{cite web |url=http://www.xydatasource.com/xy-showdatasetpage.php?datasetcode=345115&dsid=80 |title=XYdatasource - Fundamental Research Data at Your Fingertips |access-date=2009-10-02 |url-status=dead |archive-url=https://web.archive.org/web/20110718132921/http://www.xydatasource.com/xy-showdatasetpage.php?datasetcode=345115&dsid=80 |archive-date=2011-07-18 }}</ref> This makes various liquids used in everyday life, such as some foods, lotions, cleaners, and paints, viscous when stationary, but much more free-flowing when even slight shear is applied by stirring or shaking, pouring, wiping, or brushing. This property is named pseudoplasticity or [[shear thinning]]; the study of such matters is called [[rheology]].{{citation needed|date=May 2023}}

{| class="wikitable" style="text-align:right;"
|+ Viscosity of Welan gum<ref>{{cite web | url=http://www.xydatasource.com/xy-showdatasetpage.php?datasetcode=45615&dsid=76&searchtext=polysaccharide | archive-url=https://web.archive.org/web/20110718133252/http://www.xydatasource.com/xy-showdatasetpage.php?datasetcode=45615&dsid=76&searchtext=polysaccharide | archive-date=18 July 2011 | title=XYdatasource - Fundamental Research Data at Your Fingertips }}</ref>
|-
! Shear rate (rpm)
! Viscosity ([[centipoise|cP]] or mPa⋅s)
|-
| 0.3
| 23330
|-
| 0.5
| 16000
|-
| 1
| 11000
|-
| 2
| 5500
|-
| 4
| 3250
|-
| 5
| 2900
|-
| 10
| 1700
|-
| 20
| 900
|-
| 50
| 520
|-
| 100
| 310
|}

Aqueous solutions of the polysaccharide alone have a curious behavior when stirred: after stirring ceases, the solution initially continues to swirl due to momentum, then slows to a standstill due to viscosity and reverses direction briefly before stopping. This recoil is due to the elastic effect of the polysaccharide chains, previously stretched in solution, returning to their relaxed state.

Cell-surface polysaccharides play diverse roles in bacterial [[ecology]] and [[physiology]]. They serve as a barrier between the [[cell wall]] and the environment, mediate host-pathogen interactions. Polysaccharides also play an important role in formation of [[biofilm]]s and the structuring of complex life forms in bacteria like ''[[Myxococcus xanthus]]''<ref name="Modulation of bacterial multicellul"/>''.''

These polysaccharides are synthesized from [[nucleotide]]-activated precursors (called [[nucleotide sugar]]s) and, in most cases, all the enzymes necessary for biosynthesis, assembly and transport of the completed polymer are encoded by genes organized in dedicated clusters within the genome of the [[organism]]. [[Lipopolysaccharide]] is one of the most important cell-surface polysaccharides, as it plays a key structural role in outer membrane integrity, as well as being an important mediator of host-pathogen interactions.

The enzymes that make the ''A-band'' (homopolymeric) and ''B-band'' (heteropolymeric) O-antigens have been identified and the [[metabolic pathway]]s defined.<ref>{{cite journal | vauthors = Guo H, Yi W, Song JK, Wang PG | title = Current understanding on biosynthesis of microbial polysaccharides | journal = Current Topics in Medicinal Chemistry | volume = 8 | issue = 2 | pages = 141–51 | year = 2008 | pmid = 18289083 | doi = 10.2174/156802608783378873 }}</ref> The exopolysaccharide alginate is a linear copolymer of β-1,4-linked {{sc|D}}-mannuronic acid and {{sc|L}}-guluronic acid residues, and is responsible for the mucoid phenotype of late-stage cystic fibrosis disease. The ''pel'' and ''psl'' loci are two recently discovered gene clusters that also encode [[exopolysaccharide]]s found to be important for biofilm formation. [[Rhamnolipid]] is a biosurfactant whose production is tightly regulated at the [[Transcription (genetics)|transcription]]al level, but the precise role that it plays in disease is not well understood at present. Protein [[glycosylation]], particularly of [[pilin]] and [[flagellin]], became a focus of research by several groups from about 2007, and has been shown to be important for adhesion and invasion during bacterial infection.<ref name=Cornelis>{{cite book | veditors = Cornelis P | title = Pseudomonas: Genomics and Molecular Biology | edition = 1st | publisher = Caister Academic Press | year = 2008 | url=http://www.horizonpress.com/pseudo | isbn = 978-1-904455-19-6}}</ref>