Production of hydrogen sulfide (H2S) is a distinguishing feature of which aerobic gram-positive bacillus?

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

Answer: (B)

Hydrogen sulfide production on culture is a handy differentiating test for aerobic gram-positive bacilli. When these organisms are grown on media that reveal H2S (such as SIM or TSI with iron), a positive result appears as a black precipitate where sulfide reacts with iron. Among the options, Erysipelothrix rhusiopathiae is the one that reliably produces hydrogen sulfide, so a positive H2S result helps point to this organism. The other listed gram-positive rods generally do not yield H2S under standard testing, so they don’t share this feature. In practice, a Gram-positive, aerobic rod with a positive H2S reaction would steer the identification toward Erysipelothrix, with confirmation from additional biochemical and colony-characteristic tests.