Bacteria producing clear colonies with black centers on Hektoen-enteric agar were isolated from a stool culture. Which of the following is true?

• A. The bacteria are lactose fermenters.
• B. Colony morphology is suggestive of Salmonella.
• C. Colony morphology is suggestive of Shigella.
• D. Colony morphology is suggestive of Yersinia enterocolitica.

Answer: (B)

On Hektoen-enteric agar, black centers indicate hydrogen sulfide production, detected by ferric ammonium citrate in the medium. Clear colonies mean the organism is not fermenting lactose. So a non-lactose-fermenting organism that also produces H2S will appear as colorless (or green-blue) colonies with a prominent black center. Among common stool pathogens, Salmonella is the classic H2S producer that yields this exact appearance. Shigella also non-ferments lactose but does not produce H2S, so it would lack the black center. Yersinia enterocolitica typically does not show the characteristic black-centered colonies either. Therefore, the observed colony morphology strongly points to Salmonella.