Bacterial capsule

The bacterial capsule is a very large structure of 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 outer red layer in this diagram is the capsule, which is distinct from the cell envelope. This bacterium is gram-positive, as its cell envelope comprises a single cell membrane (orange) and a thick peptidoglycan containing cell wall (purple).

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.

A bacterial capsule has a semi-ridged border that follows the contour of the cell. The capsule excludes India Ink when dyed. A slime layer is a non-ridged matrix that is easily deformed and is not able to exclude India Ink. Biofilms are composed of many cells and their outer barriers. The primary functions of both capsules and slime layers are for protection and adhesion.

Composition
Further information: Polysaccharide § Bacterial capsular polysaccharides

Most bacterial capsules are composed of polysaccharide,[4] 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.[5]

Function

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. Capsules also contain water which protects the bacteria against desiccation. 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,[19] though not a bacterium, has a similar capsule.[20][21]

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.[22]: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 acid fuschin.
  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,[23][24] S. pneumoniae, and N. meningitidis[25]). 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.[26]

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See also

References
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  4. "bacterial capsule" at Dorland's Medical Dictionary
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  15. Deretic V, Dikshit R, Konyecsni WM, Chakrabarty AM, Misra TK (March 1989). "The algR gene, which regulates mucoidy in Pseudomonas aeruginosa, belongs to a class of environmentally responsive genes". Journal of Bacteriology. 171 (3): 1278–83. doi:10.1128/jb.171.3.1278-1283.1989. PMC 209741. PMID 2493441.
  16. Gibson DL, White AP, Snyder SD, Martin S, Heiss C, Azadi P, et al. (November 2006). "Salmonella produces an O-antigen capsule regulated by AgfD and important for environmental persistence". Journal of Bacteriology. 188 (22): 7722–30. doi:10.1128/JB.00809-06. PMC 1636306. PMID 17079680.
  17. Hamaguchi S, Zafar MA, Cammer M, Weiser JN (March 2018). "Capsule Prolongs Survival of Streptococcus pneumoniae during Starvation". Infection and Immunity. 86 (3). doi:10.1128/IAI.00802-17. PMC 5820961. PMID 29311231.
  18. Hyams C, Camberlein E, Cohen JM, Bax K, Brown JS (February 2010). "The Streptococcus pneumoniae capsule inhibits complement activity and neutrophil phagocytosis by multiple mechanisms". Infection and Immunity. 78 (2): 704–15. doi:10.1128/IAI.00881-09. PMC 2812187. PMID 19948837.
  19. O'Meara TR, Alspaugh JA (July 2012). "The Cryptococcus neoformans capsule: a sword and a shield". Clinical Microbiology Reviews. 25 (3): 387–408. doi:10.1128/CMR.00001-12. PMC 3416491. PMID 22763631.
  20. Gates MA, Thorkildson P, Kozel TR (April 2004). "Molecular architecture of the Cryptococcus neoformans capsule". Molecular Microbiology. 52 (1): 13–24. doi:10.1111/j.1365-2958.2003.03957.x. PMID 15049807.
  21. Casadevall A, Coelho C, Cordero RJ, Dragotakes Q, Jung E, Vij R, Wear MP (December 2019). "Cryptococcus neoformans". Virulence. 10 (1): 822–831. doi:10.1080/21505594.2018.1431087. PMC 6779390. PMID 29436899.
  22. Rudolph K (1996). "Chapter 3: Pseudomonas synringae pathovars". In Singh RP, Kohmoto K, Singh US (eds.). Pathogenesis & Host Specificity in Plant Diseases. Volume 1: Prokaryotes (1st ed.). Amsterdam: Elsevier Science. ISBN 978-0-08-098473-5.
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  25. Tzeng YL, Thomas J, Stephens DS (September 2016). "Regulation of capsule in Neisseria meningitidis". Critical Reviews in Microbiology. 42 (5): 759–72. doi:10.3109/1040841X.2015.1022507. PMC 4893341. PMID 26089023.
  26. Goldblatt D (January 2000). "Conjugate vaccines". Clinical and Experimental Immunology. 119 (1): 1–3. doi:10.1046/j.1365-2249.2000.01109.x. PMC 1905528. PMID 10671089.
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