Bacterial Capsule: Structure,Importance and examples of capsulated bacteria

Capsule (also known as K antigen) is a major virulence factor of bacteria, e.g. all of the principal pathogens which cause pneumonia and meningitis, including Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis, Klebsiella pneumoniae,  Escherichia coli, and group B streptococci have polysaccharide capsules on their surface. Nonencapsulated mutants of these organisms are avirulent.

Capsule is a 

  • Gelatinous layer covering the entire bacterium
  • Composed of polysaccharide (i.e. poly: many, saccharide: sugar). These polymers are composed of repeating oligosaccharide units of two to four monosaccharides.

    Bacterial Capsule  Image source: ASM
    Bacterial Capsule

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

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

  • The sugar components 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 anti-phagocytic. They limit the ability of phagocytes to engulf the bacteria. The smooth nature and negative charge of the capsule prevents the phagocyte from adhering to and engulfing the bacterial cell. If a pathogenic bacteria lose capsule (by mutation), they wont be able to cause disease (i.e. loses disease causing capacity).
  2. Saving engulfed bacteria from the action of neutrophil: Bacterial capsule prevents the direct access of lysosome contents with the bacterial cell, preventing their killing.
  3. Prevention of complement-mediated bacterial cell lysis.
  4. Protection of anaerobes from oxygen toxicity.
  5. Identification of bacteria:
    Anti-phagocytic nature of Bacterial capsule
    Anti-phagocytic nature of Bacterial capsule
    1. Using specific antiserum against capsular polysaccharide.  E.g. Quellung reaction
    2. Colony characteristics in culture media: Bacteria with capsules form smooth (S) colonies while those without capsules form rough (R) colonies. A given bacterial species may undergo a phenomenon called S-R variation whereby the cell loses the ability to form a capsule. Some capsules are very large and absorb water; bacteria with this type of capsule (e.g., Klebsiella pneumoniae) form mucoid (M) colonies.
  6. Development of Vaccines: Capsular polysaccharides are used as the antigens in certain vaccines. For examples:
    • Polyvalent (23 serotypes) polysaccharide vaccine of Streptococcus pneumoniae capsule.
    • Polyvalent (4 serotypes) vaccine of Neisseria meningitidis capsule.
    • A monovalent vaccine made up of capsular material from Haemophilus influenzae.
  7. Initiation of infection: Capsules helps the organism to adhere to host cells.The capsule also facilitates and maintains bacterial colonization of biologic (e.g. teeth) and inanimate (e.g. prosthetic heart valves) surfaces through formation of biofilms.
  8. Receptors for Bacteriophages.

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

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