Which test is used in vitro to detect toxigenic strains of Corynebacterium diphtheriae?

• A. D-test
• B. Elek test
• C. Nagler test
• D. Modified Hodge test

Answer: (B)

The key idea is to demonstrate toxin production by Corynebacterium diphtheriae in the lab. The Elek immunodiffusion test does this by using an agar plate containing diphtheria antitoxin. A bacterial isolate is streaked perpendicularly to a line of antitoxin. If the organism secretes diphtheria toxin, the toxin diffuses into the agar and meets the antitoxin, forming a visible precipitin line where they interact. This line indicates toxigenicity, meaning the strain can produce the toxin. If the isolate does not produce toxin, no precipitin line appears.

The other tests are used for different purposes: the D-test detects inducible antibiotic resistance patterns, not toxin production; the Nagler test detects lecithinase activity of Clostridium perfringens; and the Modified Hodge test identifies carbapenemase production in Gram-negative bacteria. Therefore, the Elek test is the one specifically designed to assess toxigenic potential in vitro.