Elek's gel precipitation test is performed to determine which property of a Corynebacterium diphtheriae isolate?

• A Serotyping based on somatic O antigens
• B Beta lysin conversion mediated by a temperate bacteriophage
• C Bacteriocin production for epidemiological typing
• D Toxigenicity, by demonstration of precipitin lines of identity with a reference antitoxin ✓

Explanation

In Elek's test a filter paper strip impregnated with diphtheria antitoxin is laid into molten serum agar and the test strain is streaked across it. Toxin diffusing from a toxigenic strain meets the antitoxin and forms visible precipitin arcs that fuse with those of a control toxigenic strain (lines of identity). Phage conversion explains why a strain becomes toxigenic but is not what the test measures.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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