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From Wikipedia, the free encyclopedia

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.



It is usually composed of polysaccharides,[1] but could 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.


Due to the fact that the capsule helps to protect bacteria against phagocytosis, it is considered a virulence factor. 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. Further than that, bacterial capsules allow bacteria to adhere to surfaces and other cells.


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.[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:

"Some Killers Have Pretty Nice Capsules"

Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Pseudomonas aeruginosa, Neisseria meningitidis, and the yeast Cryptococcus neoformans.

Use in vaccination

Vaccination using capsular material is effective against some organisms (e.g., H. influenzae type b and S. pneumoniae).

See also




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