Bacterial capsule

The outer red layer in this diagram is the capsule, which is distinct from the cell envelope. This bacteria is Gram-positive, as its cell envelope comprises a single plasma membrane (orange) and a thick peptidoglycan-containing cell wall (purple).

The cell capsule is a very large structure of some prokaryotic cells, such as bacterial cells. It is a polysaccharide layer that lies outside the cell envelope of bacteria, 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.

The capsule—which can be found in both Gram-negative bacteria and Gram-positive bacteria—should not be confused with the second lipid membrane (or 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 slime layer.

Composition

It usually consists of polysaccharides,[1] but can be composed of other materials (e.g., polypeptide(D-glutamic acid) in B. anthracis). Because most capsules are so tightly packed, they are difficult to stain using standard stains because most stains cannot adhere to the capsule. For examination under the microscope, the bacteria and their background are stained darker than the capsule, which doesn't stain. When viewed, bacterial cells as well as the surface they are on, are stained dark, while the capsule remains pale or colorless and appears as a ring, or halo, around the cell.[2]

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.[3] 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. There are 14 different capsule types, which each impart their own specific antigenicity. Immunity to one capsule type does not result in immunity to the other types. Capsules also help cells adhere to surfaces.

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, though not a bacterium, has a similar capsule.[8]

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

Mnemonic

A common mnemonic used to remember some encapsulated pathogens is:

"Even Some Super Killers Have Pretty Nice Big Capsules"

Escherichia coli, Streptococcus pneumoniae, Salmonella, Klebsiella pneumoniae, Haemophilus influenzae, Pseudomonas aeruginosa, Neisseria meningitidis, Bacteroides fragilis, and the yeast Cryptococcus neoformans.

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.[9]:87
  2. 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, S. pneumoniae, and N. meningitidis).

See also

References

  1. ↑ "bacterial capsule" at Dorland's Medical Dictionary
  2. ↑ Encyclopedia Britannica [Capsules and Slime Layers].
  3. ↑ Daffé, M.; Etienne, G. (1999). "The capsule of Mycobacterium tuberculosis and its implications for pathogenicity". Tubercle and Lung Disease 79 (3): 153–69. doi:10.1054/tuld.1998.0200. PMID 10656114.
  4. ↑ "Meningococcal meningitis". Textbookofbacteriology.net. Retrieved 2014-01-22.
  5. ↑ Yoshida, K; Matsumoto, T; Tateda, K; Uchida, K; Tsujimoto, S; Yamaguchi, K (November 2000). "Role of bacterial capsule in local and systemic inflammatory responses of mice during pulmonary infection with Klebsiella pneumoniae". J. Med. Microbiol. 49 (11): 1003–10. PMID 11073154.
  6. ↑ Schouls, Leo; Van Der Heide, Han; Witteveen, Sandra; Zomer, Bert; Van Der Ende, Arie; Burger, Marina; Schot, Corrie (2008). "Two variants among Haemophilus influenzae serotype b strains with distinct bcs4, hcsA and hcsB genes display differences in expression of the polysaccharide capsule". BMC Microbiol. 8 (1): 35. doi:10.1186/1471-2180-8-35. PMC 2267795. PMID 18298818.
  7. ↑ Deretic, V; Dikshit, R; Konyecsni, WM; Chakrabarty, AM; Misra, TK (1989). "The algR gene, which regulates mucoidy in Pseudomonas aeruginosa, belongs to a class of environmentally responsive genes". Journal of Bacteriology 171 (3): 1278–1283. PMC 209741. PMID 2493441.
  8. ↑ Gates, Marcellene A.; Thorkildson, Peter; Kozel, Thomas R. (April 1390). "Molecular architecture of the Cryptococcus neoformans capsule". Mol. Microbiol. 52 (1): 13–24. doi:10.1111/j.1365-2958.2003.03957.x. PMID 15049807.
  9. ↑ Prokaryotes. Newnes. Apr 11, 1996. ISBN 978-0-08-098473-5.
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