Paper I — 2016 October (Supplementary) (2010 Scheme) — Q8
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Paper I
2016 October (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Bacterial capsule
Q82 marksShort Notes
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) surrounding the cell wall of certain bacteria, distinguishable from the more diffuse, loosely adherent glycocalyx/slime layer.
Demonstration: not well stained by ordinary dyes; best demonstrated by negative staining (e.g., India ink preparation, giving a clear halo against a dark background) or specific capsule stains (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.
Antigenic (basis for typing and vaccines) — capsular polysaccharide is often serotype-specific and forms the basis of vaccines (pneumococcal, meningococcal, H. influenzae type b conjugate vaccines) and the Quellung (capsular swelling) reaction for serotyping.
Adherence — contributes to biofilm formation and surface adherence in some organisms.
Examples of important encapsulated bacteria: Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis, Klebsiella pneumoniae, Bacillus anthracis — asplenic/hyposplenic patients are at particular risk of overwhelming infection with these organisms, since the spleen is important for their clearance.
