Enteric bacteriology – H2S production by Salmonella: Which Salmonella serotype listed is classically non–hydrogen sulfide (H2S) producing on routine differential media, aiding presumptive identification in the laboratory?

Introduction / Context: Production of hydrogen sulfide (H2S) on media such as triple sugar iron (TSI) agar or Hektoen enteric agar is a useful screening trait for enteric pathogens. Most Salmonella produce H2S, but notable exceptions help refine preliminary identification.

Given Data / Assumptions:

• Most Salmonella serotypes are H2S positive (blackening on media).
• S. Paratyphi A and S. choleraesuis are classically H2S negative.
• H2S phenotype supports, but does not by itself confirm, species/serotype identification.

Concept / Approach: Recognizing the exceptions prevents misclassification. Non–H2S-producing Salmonella can resemble Shigella colonies on some media, so additional biochemical and serologic tests are required. The well-taught pairing of S. Paratyphi A and S. choleraesuis as H2S negative is a common examination point.

Step-by-Step Solution: Recall the rule: “Most Salmonella are H2S positive.” Identify exceptions: S. Paratyphi A and S. choleraesuis. Select the combined option listing both exceptions. Plan confirmatory serology if encountered in practice.

Verification / Alternative check: Bench manuals highlight these serotypes as non–H2S producers, advising careful differentiation from Shigella.

Why Other Options Are Wrong: Choosing one alone is incomplete; “none” ignores known exceptions; S. Typhi often yields weak but present H2S.

Common Pitfalls: Overreliance on H2S alone for identification; always corroborate with antigenic typing and additional biochemicals.

Final Answer: Both (a) and (b).