bacterial capsule

{{Further|Polysaccharide#Bacterial capsular polysaccharides}}

Most bacterial capsules are composed of polysaccharide,{{DorlandsDict|two/000016823|bacterial capsule}} but some species use other materials, such as poly-D-glutamic acid in Bacillus anthracis. Because most capsules are so tightly packed, they are difficult to stain because most standard stains cannot penetrate the capsule. To visualize encapsulated bacteria using a microscope, a sample is treated with a dark stain, such as India ink. The structure of the capsule prevents the stain from penetrating the cell. When viewed,JB bacterial capsules appear as a bright halo around the cell on a dark background.{{cite encyclopedia|url= http://www.britannica.com/EBchecked/topic/48203/bacteria/39338/Capsules-and-slime-layers|encyclopedia= Encyclopædia Britannica|title=Basteria: Capsules and Slime Layers|url-status=dead|archive-url=https://web.archive.org/web/20130308143120/http://www.britannica.com/EBchecked/topic/48203/bacteria/39338/Capsules-and-slime-layers|archive-date=2013-03-08}}

The bacterial capsule serves as a shield, giving protection from toxins, and from drying out. Capsules allow adhesion to surfaces and help enable the bacteria to evade the host immune system.{{cite journal |vauthors=Santos-López A, Rodríguez-Beltrán J, San Millán Á |title=The bacterial capsule is a gatekeeper for mobile DNA |journal=PLOS Biol |volume=19 |issue=7 |pages=e3001308 |date=July 2021 |pmid=34228713 |pmc=8260180 |doi=10.1371/journal.pbio.3001308 |url= |doi-access=free }} The water content in the capsule gives the protection against drying out.

The capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease (e.g. prevents phagocytosis). The capsule can protect cells from engulfment by eukaryotic cells, such as macrophages.{{cite journal | vauthors = Daffé M, Etienne G | title = The capsule of Mycobacterium tuberculosis and its implications for pathogenicity | journal = Tubercle and Lung Disease | volume = 79 | issue = 3 | pages = 153–69 | year = 1999 | pmid = 10656114 | doi = 10.1054/tuld.1998.0200 }} A capsule-specific antibody may be required for phagocytosis to occur. They also exclude bacterial viruses and most hydrophobic toxic materials such as detergents.{{Citation needed|date=March 2016}} Immunity to one capsule type does not result in immunity to the other types. Capsules also help cells adhere to surfaces. As a group where the capsule is present they are known as polysaccharide encapsulated bacteria or encapsulated bacteria.{{cite journal | vauthors = Lindberg AA | title = Polyosides (encapsulated bacteria) | journal =Comptes Rendus de l'Académie des Sciences, Série III | volume = 322 | issue = 11 | pages = 925–32 | date = November 1999 | pmid = 10646085 | doi = 10.1016/s0764-4469(00)87188-7 | bibcode = 1999CRASG.322..925L }}

The capsule is found most commonly among gram-negative bacteria:

• Escherichia coli (in some strains)
• Neisseria meningitidis{{cite web|url=http://textbookofbacteriology.net/themicrobialworld/meningitis.html |title=Meningococcal meningitis |publisher=Textbookofbacteriology.net |access-date=2014-01-22 |url-status=dead |archive-url=https://web.archive.org/web/20140209125613/http://textbookofbacteriology.net/themicrobialworld/meningitis.html |archive-date=2014-02-09 }}{{cite journal | vauthors = Ganesh K, Allam M, Wolter N, Bratcher HB, Harrison OB, Lucidarme J, Borrow R, de Gouveia L, Meiring S, Birkhead M, Maiden MC, von Gottberg A, du Plessis M | display-authors = 6 | title = Molecular characterization of invasive capsule null Neisseria meningitidis in South Africa | journal = BMC Microbiology | volume = 17 | issue = 1 | pages = 40 | date = February 2017 | pmid = 28222677 | pmc = 5320719 | doi = 10.1186/s12866-017-0942-5 | doi-access = free }}{{cite journal | vauthors = Harrison OB, Claus H, Jiang Y, Bennett JS, Bratcher HB, Jolley KA, Corton C, Care R, Poolman JT, Zollinger WD, Frasch CE, Stephens DS, Feavers I, Frosch M, Parkhill J, Vogel U, Quail MA, Bentley SD, Maiden MC | display-authors = 6 | title = Description and nomenclature of Neisseria meningitidis capsule locus | language = en-us | journal = Emerging Infectious Diseases | volume = 19 | issue = 4 | pages = 566–73 | date = April 2013 | pmid = 23628376 | pmc = 3647402 | doi = 10.3201/eid1904.111799 }}
• Klebsiella pneumoniae{{cite journal | vauthors = Yoshida K, Matsumoto T, Tateda K, Uchida K, Tsujimoto S, Yamaguchi K | title = Role of bacterial capsule in local and systemic inflammatory responses of mice during pulmonary infection with Klebsiella pneumoniae | journal = Journal of Medical Microbiology | volume = 49 | issue = 11 | pages = 1003–10 | date = November 2000 | pmid = 11073154 | doi = 10.1099/0022-1317-49-11-1003 | doi-access = free }}{{cite journal | vauthors = Dorman MJ, Feltwell T, Goulding DA, Parkhill J, Short FL | title = Klebsiella pneumoniae Defined by density-TraDISort | journal = mBio | volume = 9 | issue = 6 | date = November 2018 | pmid = 30459193 | pmc = 6247091 | doi = 10.1128/mBio.01863-18 }}{{cite journal | vauthors = Schembri MA, Blom J, Krogfelt KA, Klemm P | title = Capsule and fimbria interaction in Klebsiella pneumoniae | journal = Infection and Immunity | volume = 73 | issue = 8 | pages = 4626–33 | date = August 2005 | pmid = 16040975 | pmc = 1201234 | doi = 10.1128/IAI.73.8.4626-4633.2005 }}
• Haemophilus influenzae{{cite journal | vauthors = Schouls L, van der Heide H, Witteveen S, Zomer B, van der Ende A, Burger M, Schot C | title = Two variants among Haemophilus influenzae serotype b strains with distinct bcs4, hcsA and hcsB genes display differences in expression of the polysaccharide capsule | journal = BMC Microbiology | volume = 8 | issue = 1 | pages = 35 | date = February 2008 | pmid = 18298818 | pmc = 2267795 | doi = 10.1186/1471-2180-8-35 | doi-access = free }}
• Pseudomonas aeruginosa{{cite journal | vauthors = Deretic V, Dikshit R, Konyecsni WM, Chakrabarty AM, Misra TK | title = The algR gene, which regulates mucoidy in Pseudomonas aeruginosa, belongs to a class of environmentally responsive genes | journal = Journal of Bacteriology | volume = 171 | issue = 3 | pages = 1278–83 | date = March 1989 | pmid = 2493441 | pmc = 209741 | doi = 10.1128/jb.171.3.1278-1283.1989 }}
• Salmonella{{cite journal | vauthors = Gibson DL, White AP, Snyder SD, Martin S, Heiss C, Azadi P, Surette M, Kay WW | display-authors = 6 | title = Salmonella produces an O-antigen capsule regulated by AgfD and important for environmental persistence | journal = Journal of Bacteriology | volume = 188 | issue = 22 | pages = 7722–30 | date = November 2006 | pmid = 17079680 | pmc = 1636306 | doi = 10.1128/JB.00809-06 }}
• Acinetobacter baumannii{{Cite journal|last1=Kenyon|first1=Johanna J.|last2=Hall|first2=Ruth M.|date=2013-04-16|editor-last=de Crécy-Lagard|editor-first=Valerie|title=Variation in the Complex Carbohydrate Biosynthesis Loci of Acinetobacter baumannii Genomes|journal=PLOS ONE|language=en|volume=8|issue=4|pages=e62160|doi=10.1371/journal.pone.0062160|issn=1932-6203|pmc=3628348|pmid=23614028|bibcode=2013PLoSO...862160K|doi-access=free}}{{Cite journal|last1=Singh|first1=Jennifer K.|last2=Adams|first2=Felise G.|last3=Brown|first3=Melissa H.|date=2019-01-09|title=Diversity and Function of Capsular Polysaccharide in Acinetobacter baumannii|journal=Frontiers in Microbiology|volume=9|pages=3301|doi=10.3389/fmicb.2018.03301|issn=1664-302X|pmc=6333632|pmid=30687280|doi-access=free}}

However, some gram-positive bacteria may also have a capsule:

• Bacillus megaterium for example, synthesizes a capsule composed of polypeptide and polysaccharides.
• Bacillus anthracis
• Streptococcus pyogenes synthesizes a hyaluronic acid capsule.
• Streptococcus pneumoniae{{cite journal | vauthors = Hamaguchi S, Zafar MA, Cammer M, Weiser JN | title = Capsule Prolongs Survival of Streptococcus pneumoniae during Starvation | journal = Infection and Immunity | volume = 86 | issue = 3 | date = March 2018 | pmid = 29311231 | pmc = 5820961 | doi = 10.1128/IAI.00802-17 }} has at least 91 different capsular serotypes.{{cite journal | vauthors = Hyams C, Camberlein E, Cohen JM, Bax K, Brown JS | title = The Streptococcus pneumoniae capsule inhibits complement activity and neutrophil phagocytosis by multiple mechanisms | journal = Infection and Immunity | volume = 78 | issue = 2 | pages = 704–15 | date = February 2010 | pmid = 19948837 | pmc = 2812187 | doi = 10.1128/IAI.00881-09 }} These serotypes are the basis for the pneumococcal vaccines.
• Streptococcus agalactiae produces a polysaccharide capsule of nine antigenic types that all contain sialic acid (Ia, Ib, II, III, IV, V, VI, VII, VIII).
• Staphylococcus epidermidis
• Staphylococcus aureus
• Lactococcus garvieae synthesizes capsular gene clusters and some time synthesizes a hyaluronic acid capsule.{{cite journal | url=https://onlinelibrary.wiley.com/doi/10.1111/jfd.13601 | doi=10.1111/jfd.13601 | title=Genotyping and phenotyping of Lactococcus garvieae isolates from fish by pulse-field gel electrophoresis (PFGE) and electron microscopy indicate geographical and capsular variations | year=2022 | last1=Rao | first1=Shreesha | last2=Chen | first2=Mei-Yun | last3=Sudpraseart | first3=Chiranan | last4=Lin | first4=Peiry | last5=Yoshida | first5=Terutoyo | last6=Wang | first6=Pei-Chi | last7=Chen | first7=Shih-Chu | journal=Journal of Fish Diseases | volume=45 | issue=6 | pages=771–781 | pmid=35235703 | bibcode=2022JFDis..45..771R | s2cid=247220475 | url-access=subscription }}

The yeast Cryptococcus neoformans,{{cite journal | vauthors = O'Meara TR, Alspaugh JA | title = The Cryptococcus neoformans capsule: a sword and a shield | journal = Clinical Microbiology Reviews | volume = 25 | issue = 3 | pages = 387–408 | date = July 2012 | pmid = 22763631 | pmc = 3416491 | doi = 10.1128/CMR.00001-12 }} though not a bacterium, has a similar capsule.{{cite journal | vauthors = Gates MA, Thorkildson P, Kozel TR | title = Molecular architecture of the Cryptococcus neoformans capsule | journal = Molecular Microbiology | volume = 52 | issue = 1 | pages = 13–24 | date = April 2004 | pmid = 15049807 | doi = 10.1111/j.1365-2958.2003.03957.x | doi-access = free }}{{cite journal | vauthors = Casadevall A, Coelho C, Cordero RJ, Dragotakes Q, Jung E, Vij R, Wear MP | title = Cryptococcus neoformans | journal = Virulence | volume = 10 | issue = 1 | pages = 822–831 | date = December 2019 | pmid = 29436899 | pmc = 6779390 | doi = 10.1080/21505594.2018.1431087 }}

Capsules too small to be seen with an ordinary microscope, such as the M protein of Streptococcus pyogenes, are called microcapsules.

1. India ink staining: the capsule appears as a clear halo around the bacterium as the ink can't penetrate the capsule.{{cite book | first = K.W.E. | last = Rudolph | chapter = Chapter 3: Pseudomonas synringae pathovars | editor-first1 = Rudra P. | editor-last1 = Singh |editor-first2 = Keisuke | editor-last2 = Kohmoto | editor-first3 = Uma S. | editor-last3 = Singh | name-list-style = vanc | title = Pathogenesis & Host Specificity in Plant Diseases | volume = 1: Prokaryotes | date = 1996 | publisher = Elsevier Science | location = Amsterdam | isbn = 978-0-08-098473-5 | edition = 1st }}{{rp|87}}
2. Maneval's capsule stain: the capsule appears as a clear halo between the pink-stained bacterium and the bluish-grey stained background. The background stain is the acidic stain Congo red (which changes color to bluish-grey due to the pH), and the pink stain is fuchsine.
3. Serological methods: Capsular material is antigenic and can be demonstrated by mixing it with a specific anticapsular serum. When examined under the microscope, the capsule appears 'swollen' due to an increase in its refractivity. This phenomenon is the basis of quellung reaction.

Vaccination using capsular material is effective against some organisms (e.g., H. influenzae type b,{{cite journal | vauthors = Satola SW, Collins JT, Napier R, Farley MM | title = Capsule gene analysis of invasive Haemophilus influenzae: accuracy of serotyping and prevalence of IS1016 among nontypeable isolates | journal = Journal of Clinical Microbiology | volume = 45 | issue = 10 | pages = 3230–8 | date = October 2007 | pmid = 17699642 | pmc = 2045354 | doi = 10.1128/JCM.00794-07 }}{{cite journal | vauthors = Watts SC, Holt KE | title = In Silico Serotyping of the Haemophilus influenzae Capsule Locus | journal = Journal of Clinical Microbiology | volume = 57 | issue = 6 | date = June 2019 | pmid = 30944197 | pmc = 6535587 | doi = 10.1128/JCM.00190-19 }} S. pneumoniae, and N. meningitidis{{cite journal | vauthors = Tzeng YL, Thomas J, Stephens DS | title = Regulation of capsule in Neisseria meningitidis | journal = Critical Reviews in Microbiology | volume = 42 | issue = 5 | pages = 759–72 | date = September 2016 | pmid = 26089023 | pmc = 4893341 | doi = 10.3109/1040841X.2015.1022507 }}). However, polysaccharides are not highly antigenic, especially in children, so many capsular vaccines contain polysaccharides conjugated with protein carriers, such as the tetanus toxoid or diphtheria toxoid. This stimulates a much more robust immune response.{{cite journal | vauthors = Goldblatt D | title = Conjugate vaccines | journal = Clinical and Experimental Immunology | volume = 119 | issue = 1 | pages = 1–3 | date = January 2000 | pmid = 10671089 | pmc = 1905528 | doi = 10.1046/j.1365-2249.2000.01109.x }}

• Bacterial cell structure
• Quellung reaction, a method to visualize capsule under a microscope

{{reflist|32em}}

{{Bacteria}}

Category:Bacteriology

Category:Prokaryotic cell anatomy