The cell capsule is a very large structure of some prokaryotic cells, such as bacterial cells. It is a layer that lies outside the cell wall of bacteria. It is a well organized layer, not easily washed off, and it can be the cause of various diseases.
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It usually consists of polysaccharides,[1] but can be composed of other materials (e.g., polypeptide in B. anthracis). Because most capsules are water soluble , they are difficult to stain using standard stains because most stains do not 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 around the cell.
The capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease (i.e. prevents phagocytosis). The capsule is slippery and fragile, so when a phagocyte tries to phagocytose the bacteria, it can slip away. A capsule-specific antibody may be required for phagocytosis to occur. Capsules also contain water which protects 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.
The capsule is found most commonly among Gram-negative bacteria:
However, some Gram-positive bacteria may also have a capsule:
- Streptococcus agalactiae produces a polysaccharide capsule of nine antigenic types that all contain sialic acid (Ia, Ib, II, III, IV, V, VI, VII, VIII).
The yeast Cryptococcus neoformans, though not a bacterium, has a similar capsule.[4]
Capsules too small to be seen with an ordinary microscope, such as the M protein of Streptococcus pyogenes, are called microcapsules.
A common mnemonic used to remember some encapsulated pathogens is:
"Even Some Killers Have Pretty Nice Capsules"
Escherichia coli, Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Pseudomonas aeruginosa, Neisseria meningitidis, and the yeast Cryptococcus neoformans.
1. India ink staining: the capsule appears as a clear halo around the bacterium as the ink can't penetrate the capusule. 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 called as capsule swelling reaction or Quellung phenomenon. 3. Special capsule staining: These techniques employ copper salts as mordants for staining of the capsule.
Vaccination using capsular material is effective against some organisms (e.g., H. influenzae type b and S. pneumoniae).
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