Which medium is used to selectively isolate Yersinia enterocolitica from clinical specimens?

• A. MacConkey agar
• B. CIN agar
• C. Blood agar
• D. Chocolate agar

Answer: (B)

Selective isolation of Yersinia enterocolitica from mixed clinical specimens is best achieved with a medium that both suppresses competing bacteria and reveals the organism’s characteristic growth. CIN agar accomplishes this with a combination of inhibitors—cefsulodin, irgasan, and novobiocin—that inhibit many other Gram-negative rods and Gram-positive bacteria, allowing Yersinia to grow more readily. In addition, it contains mannitol and a pH indicator; when Yersinia ferments mannitol, colonies take on a distinctive bull’s-eye appearance—a pink/red center with a colorless outer zone. This combination makes CIN agar the most effective choice for selective isolation from stool and other clinical samples.

In contrast, MacConkey agar is useful for grouping Gram-negative enterics and differentiating lactose fermenters but does not provide the same selective pressure or the diagnostic bull’s-eye colony morphology. Blood agar and Chocolate agar are nonselective and can be overwhelmed by competing flora, making isolation of Yersinia harder.