Paper I — 2021 May (Supplementary) (2010 Scheme) — Q13

Paper I

2021 May (Supplementary) (2010 Scheme) · 40 marks · 120 min

Question

Laboratory diagnosis of diphtheria

Q132 marksShort Notes★✓

Answer

Laboratory diagnosis of diphtheria (Corynebacterium diphtheriae):

• Sample collection: swab from beneath the edge of the pseudomembrane (not the membrane surface), collected before antibiotics wherever possible.
• Direct microscopy: Gram stain (Gram-positive club-shaped bacilli in Chinese-letter/palisade arrangement) and Albert’s or Neisser’s stain to demonstrate metachromatic (volutin) granules — a rapid presumptive clue.
• Culture: on Loeffler’s serum slope (enrichment, rapid growth and granule formation within 6–8 hours) and tellurite blood agar (selective, gives black/grey colonies, differentiating biotypes gravis/intermedius/mitis by colony morphology).
• Toxigenicity testing: Elek’s gel precipitation test — confirms whether the isolated strain actually produces diphtheria exotoxin; in-vivo guinea pig toxin test is an older alternative.
• PCR for the toxin gene (tox) is available in reference laboratories.

Related Concepts

1. 1Diphtheria

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