Which organism is most likely to produce hydrogen sulfide on triple sugar iron agar and test positive for lysine decarboxylation but negative for urease, differentiating it from Proteus species?

• A. Citrobacter freundii
• B. Proteus mirabilis
• C. Providencia rettgeri
• D. Salmonella

Answer: (D)

Hydrogen sulfide production on triple sugar iron agar shows the organism can reduce sulfur to H2S, causing the butt of the medium to turn black. Lysine decarboxylation positive means the organism has lysine decarboxylase and can decarboxylate lysine under the test conditions, typically yielding an alkaline shift after initial acid production. Urease negative indicates the organism does not hydrolyze urea to ammonia, so there’s no rapid alkaline shift seen with urease-positive bacteria.

Putting these together, a pattern of H2S production, positive lysine decarboxylation, and negative urease fits Salmonella among the common Enterobacteriaceae and helps distinguish it from Proteus, which is urease positive. Citrobacter freundii can share H2S production and urease negativity, but the lysine decarboxylation result described is more characteristic of Salmonella in typical teaching cases.