Which organism is frequently misidentified as an enteric pathogen because some isolates produce hydrogen sulfide, leading to confusion in identification?

• A. Burkholderia cepacia
• B. Burkholderia pseudomallei
• C. Pseudomonas putida
• D. Citrobacter freundii

Answer: (D)

Hydrogen sulfide production on differential media can blur the line between some enteric bacteria and true enteric pathogens. In the Enterobacteriaceae, the ability to produce H2S is used to help distinguish organisms, because it creates a visible blackening in media like the butt of a triple sugar iron test. While this trait is helpful, it is not unique to any single pathogen. Citrobacter freundii is known to produce hydrogen sulfide, so its biochemical profile can resemble that of Salmonella or other H2S-positive pathogens. When a lab relies on H2S production as a key differentiator, Citrobacter freundii can be misidentified as an enteric pathogen, leading to confusion in identification. To resolve this, additional tests that examine lactose utilization, indole production, citrate use, urease activity, and lysine decarboxylation, among others, are used to accurately separate Citrobacter from true Salmonella or other H2S producers. The other organisms listed do not commonly present the same H2S-driven misidentification pattern due to their different metabolic and biochemical profiles.