Bacterial capsule explained

The bacterial capsule is a large structure common to many bacteria.[1] It is a polysaccharide layer that lies outside the cell envelope, and is thus deemed part of the outer envelope of a bacterial cell. It is a well-organized layer, not easily washed off, and it can be the cause of various diseases.[2] [3]

The capsule—which can be found in both gram negative and gram-positive bacteria—is different from the second lipid membrane – bacterial outer membrane, which contains lipopolysaccharides and lipoproteins and is found only in gram-negative bacteria.When the amorphous viscid secretion (that makes up the capsule) diffuses into the surrounding medium and remains as a loose undemarcated secretion, it is known as a slime layer. Capsule and slime layer are sometimes summarized under the term glycocalyx.

Composition

Most bacterial capsules are composed of polysaccharide, 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, bacterial capsules appear as a bright halo around the cell on a dark background.[4]

Function

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.[5] 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.[6] A capsule-specific antibody may be required for phagocytosis to occur. They also exclude bacterial viruses and most hydrophobic toxic materials such as detergents. 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.[7]

Diversity

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

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

The yeast Cryptococcus neoformans,[22] though not a bacterium, has a similar capsule.[23] [24]

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

Demonstration of capsule

  1. India ink staining: the capsule appears as a clear halo around the bacterium as the ink can't penetrate the capsule.[25]
  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.

Use in vaccination

Vaccination using capsular material is effective against some organisms (e.g., H. influenzae type b,[26] [27] S. pneumoniae, and N. meningitidis[28]). 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.[29]

See also

Notes and References

  1. Book: Peterson. Johnny W. . vanc . Bacterial Pathogenesis. Medical Microbiology. University of Texas Medical Branch at Galveston. 17 January 2018. 1996. 21413346 . 9780963117212 .
  2. Gao S, Lewis GD, Ashokkumar M, Hemar Y . Inactivation of microorganisms by low-frequency high-power ultrasound: 1. Effect of growth phase and capsule properties of the bacteria . Ultrasonics Sonochemistry . 21 . 1 . 446–53 . January 2014 . 23835398 . 10.1016/j.ultsonch.2013.06.006 . 24149924 .
  3. Hathaway LJ, Grandgirard D, Valente LG, Täuber MG, Leib SL . Streptococcus pneumoniae capsule determines disease severity in experimental pneumococcal meningitis . Open Biology . 6 . 3 . 150269 . March 2016 . 27009189 . 4821241 . 10.1098/rsob.150269 .
  4. Encyclopedia: Basteria: Capsules and Slime Layers. Encyclopædia Britannica. dead. https://web.archive.org/web/20130308143120/http://www.britannica.com/EBchecked/topic/48203/bacteria/39338/Capsules-and-slime-layers. 2013-03-08.
  5. Santos-López A, Rodríguez-Beltrán J, San Millán Á . The bacterial capsule is a gatekeeper for mobile DNA . PLOS Biol . 19 . 7 . e3001308 . July 2021 . 34228713 . 8260180 . 10.1371/journal.pbio.3001308 . free .
  6. Daffé M, Etienne G . The capsule of Mycobacterium tuberculosis and its implications for pathogenicity . Tubercle and Lung Disease . 79 . 3 . 153–69 . 1999 . 10656114 . 10.1054/tuld.1998.0200 .
  7. Lindberg AA . Polyosides (encapsulated bacteria) . Comptes Rendus de l'Académie des Sciences, Série III . 322 . 11 . 925–32 . November 1999 . 10646085 . 10.1016/s0764-4469(00)87188-7 . 1999CRASG.322..925L .
  8. Web site: Meningococcal meningitis . Textbookofbacteriology.net . 2014-01-22 . dead . https://web.archive.org/web/20140209125613/http://textbookofbacteriology.net/themicrobialworld/meningitis.html . 2014-02-09 .
  9. 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 . 6 . Molecular characterization of invasive capsule null Neisseria meningitidis in South Africa . BMC Microbiology . 17 . 1 . 40 . February 2017 . 28222677 . 5320719 . 10.1186/s12866-017-0942-5 . free .
  10. 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 . 6 . Description and nomenclature of Neisseria meningitidis capsule locus . en-us . Emerging Infectious Diseases . 19 . 4 . 566–73 . April 2013 . 23628376 . 3647402 . 10.3201/eid1904.111799 .
  11. Yoshida K, Matsumoto T, Tateda K, Uchida K, Tsujimoto S, Yamaguchi K . Role of bacterial capsule in local and systemic inflammatory responses of mice during pulmonary infection with Klebsiella pneumoniae . Journal of Medical Microbiology . 49 . 11 . 1003–10 . November 2000 . 11073154 . 10.1099/0022-1317-49-11-1003 . free .
  12. Dorman MJ, Feltwell T, Goulding DA, Parkhill J, Short FL . Klebsiella pneumoniae Defined by density-TraDISort . mBio . 9 . 6 . November 2018 . 30459193 . 6247091 . 10.1128/mBio.01863-18 .
  13. Schembri MA, Blom J, Krogfelt KA, Klemm P . Capsule and fimbria interaction in Klebsiella pneumoniae . Infection and Immunity . 73 . 8 . 4626–33 . August 2005 . 16040975 . 1201234 . 10.1128/IAI.73.8.4626-4633.2005 .
  14. Schouls L, van der Heide H, Witteveen S, Zomer B, van der Ende A, Burger M, Schot C . Two variants among Haemophilus influenzae serotype b strains with distinct bcs4, hcsA and hcsB genes display differences in expression of the polysaccharide capsule . BMC Microbiology . 8 . 1 . 35 . February 2008 . 18298818 . 2267795 . 10.1186/1471-2180-8-35 . free .
  15. Deretic V, Dikshit R, Konyecsni WM, Chakrabarty AM, Misra TK . The algR gene, which regulates mucoidy in Pseudomonas aeruginosa, belongs to a class of environmentally responsive genes . Journal of Bacteriology . 171 . 3 . 1278–83 . March 1989 . 2493441 . 209741 . 10.1128/jb.171.3.1278-1283.1989 .
  16. Gibson DL, White AP, Snyder SD, Martin S, Heiss C, Azadi P, Surette M, Kay WW . 6 . Salmonella produces an O-antigen capsule regulated by AgfD and important for environmental persistence . Journal of Bacteriology . 188 . 22 . 7722–30 . November 2006 . 17079680 . 1636306 . 10.1128/JB.00809-06 .
  17. Kenyon. Johanna J.. Hall. Ruth M.. 2013-04-16. de Crécy-Lagard. Valerie. Variation in the Complex Carbohydrate Biosynthesis Loci of Acinetobacter baumannii Genomes. PLOS ONE. en. 8. 4. e62160. 10.1371/journal.pone.0062160. 1932-6203. 3628348. 23614028. 2013PLoSO...862160K. free.
  18. Singh. Jennifer K.. Adams. Felise G.. Brown. Melissa H.. 2019-01-09. Diversity and Function of Capsular Polysaccharide in Acinetobacter baumannii. Frontiers in Microbiology. 9. 3301. 10.3389/fmicb.2018.03301. 1664-302X. 6333632. 30687280. free.
  19. Hamaguchi S, Zafar MA, Cammer M, Weiser JN . Capsule Prolongs Survival of Streptococcus pneumoniae during Starvation . Infection and Immunity . 86 . 3 . March 2018 . 29311231 . 5820961 . 10.1128/IAI.00802-17 .
  20. Hyams C, Camberlein E, Cohen JM, Bax K, Brown JS . The Streptococcus pneumoniae capsule inhibits complement activity and neutrophil phagocytosis by multiple mechanisms . Infection and Immunity . 78 . 2 . 704–15 . February 2010 . 19948837 . 2812187 . 10.1128/IAI.00881-09 .
  21. Genotyping and phenotyping of Lactococcus garvieae isolates from fish by pulse-field gel electrophoresis (PFGE) and electron microscopy indicate geographical and capsular variations . 10.1111/jfd.13601 . 2022 . Rao . Shreesha . Chen . Mei-Yun . Sudpraseart . Chiranan . Lin . Peiry . Yoshida . Terutoyo . Wang . Pei-Chi . Chen . Shih-Chu . Journal of Fish Diseases . 45 . 6 . 771–781 . 35235703 . 247220475 .
  22. O'Meara TR, Alspaugh JA . The Cryptococcus neoformans capsule: a sword and a shield . Clinical Microbiology Reviews . 25 . 3 . 387–408 . July 2012 . 22763631 . 3416491 . 10.1128/CMR.00001-12 .
  23. Gates MA, Thorkildson P, Kozel TR . Molecular architecture of the Cryptococcus neoformans capsule . Molecular Microbiology . 52 . 1 . 13–24 . April 2004 . 15049807 . 10.1111/j.1365-2958.2003.03957.x . free .
  24. Casadevall A, Coelho C, Cordero RJ, Dragotakes Q, Jung E, Vij R, Wear MP . Cryptococcus neoformans . Virulence . 10 . 1 . 822–831 . December 2019 . 29436899 . 6779390 . 10.1080/21505594.2018.1431087 .
  25. Book: Rudolph, K.W.E. . Chapter 3: Pseudomonas synringae pathovars . Rudra P. . Singh . Keisuke . Kohmoto . Uma S. . Singh . vanc . Pathogenesis & Host Specificity in Plant Diseases . 1: Prokaryotes . 1996 . Elsevier Science . Amsterdam . 978-0-08-098473-5 . 1st .
  26. Satola SW, Collins JT, Napier R, Farley MM . Capsule gene analysis of invasive Haemophilus influenzae: accuracy of serotyping and prevalence of IS1016 among nontypeable isolates . Journal of Clinical Microbiology . 45 . 10 . 3230–8 . October 2007 . 17699642 . 2045354 . 10.1128/JCM.00794-07 .
  27. Watts SC, Holt KE . In Silico Serotyping of the Haemophilus influenzae Capsule Locus . Journal of Clinical Microbiology . 57 . 6 . June 2019 . 30944197 . 6535587 . 10.1128/JCM.00190-19 .
  28. Tzeng YL, Thomas J, Stephens DS . Regulation of capsule in Neisseria meningitidis . Critical Reviews in Microbiology . 42 . 5 . 759–72 . September 2016 . 26089023 . 4893341 . 10.3109/1040841X.2015.1022507 .
  29. Goldblatt D . Conjugate vaccines . Clinical and Experimental Immunology . 119 . 1 . 1–3 . January 2000 . 10671089 . 1905528 . 10.1046/j.1365-2249.2000.01109.x .

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