Edwardsiella tarda is occasionally isolated in stool and can be biochemically confused with which genus?

• A. Enterohemorrhagic E. coli
• B. Vibrio cholerae
• C. Yersinia enterocolitica
• D. Salmonella

Answer: (D)

Edwardsiella tarda and Salmonella share several common biochemical traits that labs use to identify Enterobacteriaceae from stool. They are both gram-negative rods that do not ferment lactose on standard media and both can produce hydrogen sulfide in tests, with oxidase reactions that are negative. Because of this overlap, an isolate of Edwardsiella tarda can be mistaken for Salmonella on routine biochemical panels. Distinguishing them reliably requires additional tests or methods beyond the basic panel, such as extended biochemical testing or molecular identification. The other genera have characteristic features that usually separate them in standard workups (for example, Vibrio species are oxidase positive and typically have different colony and biochemical profiles), so they’re less likely to be confused in this context.