The Elek (modified Elek) test for Corynebacterium diphtheriae assesses toxin production. Which component serves as the reference in the test and what does a precipitation line indicate?

• A Antitoxin is incorporated in the agar; precipitation line indicates in-gel diffusion toxin-antitoxin complex
• B Antitoxin is applied as a central strip; precipitation lines from test organisms indicate toxin production ✓
• C Test organism is incubated with tellurite medium; black colonies indicate toxin production
• D Guinea pig skin is injected; erythema indicates positive toxin production (in vivo Elek)

Explanation

In the Elek immunodiffusion test, a filter paper strip soaked in diphtheria antitoxin is placed in the centre of a special agar plate. Test strains and control strains (known toxin-producers) are streaked perpendicularly to the strip. If the organism produces diphtheria toxin, it diffuses through the agar and forms a visible precipitation line (arc of identity) where it meets the diffusing antitoxin — a positive toxigenicity result. Lines of identity with the positive control confirm the same toxin. The in vivo skin test (intradermal inoculation in guinea pigs) was an older method; tellurite identifies C. diphtheriae colonies but not toxigenicity.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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