Bacterial Capsule

Capsule is a 

• Gelatinous layer covering the entire bacterium
• Composed of polysaccharide (i.e. poly: Many, Saccharide: Means sugar)

Capsule is located immediately exterior to the murein layer of gram-positive bacteria and the outer membrane of gram-negative bacteria.

Bacterial Capsule
Image source: ASM

(Exception: The capsule of Bacillus anthracis is composed of polymerized D-glutamic acid)

• The sugar component of polysaccharide varies within the species of bacteria, which determines their serologic types. Example: Streptococcus pneumoniae has 84 different serologic types discovered so far.

Importance of Bacterial Capsule

1. Virulence determinants: Capsules are antiphagocytic. They limit the ability of phagocytes to engulf the bacteria. If a pathogenic bacteria lose capsule (by mutation), they wont be able to cause disease (i.e. changes to nonpathogenic bacteria).
2. Identification of bacteria:
   1. Using specific antiserum against capsular polysaccharide.  E.g. Quellung reaction
   2. Colony characteristics in culture media: Capsulated organisms forms mucoid colonies
3. Development of Vaccines: Capsular polysaccharides are used as the antigens in certain vaccines. E.g. The purified capsular polysaccharides of 23 types of S. pneumoniae are present in current vaccine.
4. Initiation of infection: Capsules helps the organism to adhere to host cells.The capusle also facilitates and maintains bacterial colonization of biologic (e.g. teeth) and inanimate (e.g. prosthetic heart valves) surfaces through formation of biofilms.

Examples of Capsulated bacteria/yeasts:

Mneomonics to remember capsulated bacteria– Some Killers Have Pretty Nice Capsule

1. Streptococcus pneumoniae
2. Klebsiella pneumoniae
3. Haemophilus influenzae
4. Pseudomonas aeruginosa
5. Neisseria meningitidis
6. Cryptococcus neoformans