Which medium is used to help recover Yersinia enterocolitica from stool cultures?

• A. HEK O2 24h
• B. CIN O2 30C 24h
• C. SMAC O2 24h
• D. CAMP microaerophilic (N2) 42C 48h

Answer: (B)

Yersinia enterocolitica is best recovered from stool using a medium that is both selective and differential for this organism. CIN agar (Cefsulodin-Irgasan-Novobiocin) achieves this by inhibiting many competing bacteria present in stool, while providing a visible differential for Yersinia. The medium contains mannitol and a pH indicator, so colonies of Yersinia develop a distinctive “bull’s-eye” appearance as they ferment mannitol, making them easy to recognize among other flora.

Incubation at around 30°C in normal oxygen (aerobic) conditions for about a day is standard because Yersinia grows well at this cooler, non–body-temperature range, and the colonies become readily apparent within 24 hours. This combination—CIN’s selective antibiotics plus its differential substrate, with a 30°C, aerobic, 24-hour incubation—facilitates the recovery of Yersinia from stool.

Other media listed aren’t ideal for this purpose: HEK is more suited for differentiating Salmonella and Shigella and isn’t selective for Yersinia; SMAC targets E. coli O157 and won’t reliably reveal Yersinia; and CAMP microaerophilic conditions at 42°C are used for Campylobacter rather than Yersinia.