Stool culture shows gram-negative bacilli that are oxidase negative, citrate negative, indole negative; urease and ONPG positive; acid/acid in TSI w/o gas or H2S; which organism?

• A. Salmonella Paratyphi
• B. Shigella dysenteriae
• C. Vibrio parahaemolyticus
• D. Yersinia enterocolitica

Answer: (D)

Think about how a combination of biochemical tests patterns points to a specific enteric organism. The isolate is a gram-negative rod that is oxidase negative, which already rules out oxidase-positive Vibrios and similar organisms. The test showing citrate utilization as negative helps exclude those that require citrate as a carbon source. Indole negative narrows possibilities to a subset of non-fermenters like Shigella, Salmonella, and Yersinia.

The urease result is decisive here: Shigella and Salmonella are typically urease negative, while Yersinia enterocolitica is urease positive. A positive ONPG test indicates beta-galactosidase activity, suggesting some lactose-related metabolism; many true non-lactose fermenters (like Shigella and Salmonella) are ONPG negative, whereas this pattern can be seen with Yersinia in this panel.

Finally, the acid/acid (A/A) result in the TSI test without gas or hydrogen sulfide points to fermentation of sugars beyond glucose without gas production, which aligns with Yersinia’s profile in this context.

Putting it all together, the combination of oxidase negative, urease positive, ONPG positive, indole negative, citrate negative, and an A/A TSI without gas best fits Yersinia enterocolitica rather than the other organisms listed. Clinically, Yersinia enterocolitica is a known cause of gastroenteritis and can present with fluctuating lactose utilization and urease activity, helping distinguish it from the others.