The production of H2S is one characteristic used to differentiate which of the aerobic gram-positive bacilli?

• A. Corynebacterium
• B. Erysipelothrix
• C. Lactobacillus
• D. Nocardia

Answer: (B)

Hydrogen sulfide production is a diagnostic trait used to tell apart certain aerobic gram-positive rods in the lab. When organisms that produce H2S are grown on a medium like triple sugar iron agar, the sulfide reacts with iron in the medium to form a black precipitate along the stab line. Among the listed aerobic gram-positive bacilli, the one that typically shows this H2S production is Erysipelothrix rhusiopathiae, so the medium darkens due to ferrous sulfide. The other organisms—Corynebacterium, Lactobacillus, and Nocardia—do not characteristically produce H2S in this test, so they won’t create the same blackening. This makes H2S production a useful feature for distinguishing Erysipelothrix from the others in practical identification.