The Elek's gel precipitation test, used for demonstration of toxigenicity of Corynebacterium diphtheriae, is based on which principle?
- A Latex particles coated with diphtheria toxoid agglutinated by patient antibody
- B Neutralisation of cytopathic effect in tissue culture by specific antisera
- C Agar gel immunodiffusion between diphtheria antitoxin and exotoxin produced by the test strain ✓
- D Complement-mediated lysis of sensitised erythrocytes
Correct answer: C. Agar gel immunodiffusion between diphtheria antitoxin and exotoxin produced by the test strain
Explanation
In the Elek test, a filter paper strip soaked in diphtheria antitoxin is incorporated into agar, and the test culture is streaked perpendicular to it. Toxin diffusing from the culture meets the antitoxin and forms a visible line of precipitation that fuses with a line from a known positive control strain, confirming toxigenicity. Option B describes a virus neutralisation assay, and options A and D are agglutination and complement fixation systems respectively, none of which applies here.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
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