In the Elek's gel precipitation test used for diphtheria toxin detection, what is the principle and what does a positive result look like?
- A Antitoxin-impregnated filter paper is placed perpendicularly on agar; toxin diffusing from colonies meets antitoxin and forms precipitin lines at 45° ✓
- B Bacterial colonies are lysed by antibody, and zones of clearing indicate toxin production
- C Toxin producing colonies turn yellow on tellurite agar indicating cystinase activity
- D Agglutination of sensitised sheep RBCs by toxin-antibody complexes forms visible lattices
Correct answer: A. Antitoxin-impregnated filter paper is placed perpendicularly on agar; toxin diffusing from colonies meets antitoxin and forms precipitin lines at 45°
Explanation
Elek's in vitro immunodiffusion test: a filter paper strip soaked in diphtheria antitoxin is laid across the centre of serum agar. Test and control (known toxigenic C. diphtheriae) strains are streaked perpendicular to the strip. Toxin and antitoxin diffuse toward each other; where they meet at optimal concentrations, precipitin lines form at approximately 45° to the strip. Identity (fusion of lines from test and control) confirms toxin production. Tellurite agar is used for isolation (grey-black colonies due to tellurite reduction), not toxin detection.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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