Question
Bacterial capsule
Answer
The bacterial capsule is a well-defined, discrete gelatinous layer of polysaccharide (or, rarely, polypeptide — e.g., the poly-D-glutamic acid capsule of Bacillus anthracis) that surrounds the cell wall of certain bacteria, distinguishable from the more diffuse, loosely adherent glycocalyx/slime layer.
Demonstration: not stained by ordinary stains (it does not take up basic dyes well); best demonstrated by negative staining techniques (e.g., India ink preparation, where the capsule appears as a clear halo against a dark background, classically used for Cryptococcus neoformans in CSF, and applicable to bacterial capsules too) or specific capsule stains (e.g., Quellung reaction).
Functions/significance:
- Antiphagocytic — the major virulence function; the capsule physically prevents complement (C3b) and phagocytic cell receptors from making effective contact with the underlying cell wall, allowing the organism to resist phagocytosis and survive in blood/tissue. Encapsulated organisms are therefore typically more virulent than their unencapsulated counterparts.
- Antigenic (basis for typing and vaccines) — capsular polysaccharide is often serotype-specific and forms the basis of vaccines (e.g., pneumococcal polysaccharide/conjugate vaccines, meningococcal and H. influenzae type b conjugate vaccines) and of the Quellung (capsular swelling) reaction, used to serotype capsulated organisms with specific antisera.
- Adherence — contributes to biofilm formation and adherence to surfaces/medical devices in some organisms.
Examples of important encapsulated bacteria: Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis, Klebsiella pneumoniae, Bacillus anthracis — patients who are asplenic or hyposplenic are at particular risk of overwhelming infection with encapsulated organisms, since the spleen is important in clearing them.