Which organism produces bull's-eye colonies on CIN agar at about 48 hours, aiding its identification?

• A. Yersinia enterocolitica
• B. Escherichia coli
• C. Klebsiella pneumoniae
• D. Salmonella enterica

Answer: (A)

On CIN agar, a bull's-eye colony is a classic clue for Yersinia enterocolitica. CIN is designed to be selective for Yersinia with antibiotics (cefsulodin, irgasan, novobiocin) that suppress many other organisms, while the medium also contains mannitol and a pH indicator. Y. enterocolitica ferments mannitol and produces an acidic center, which appears red, surrounded by a pale halo from growth outside the center. This distinctive red center with a clear surrounding zone gives the bull's-eye look. The colonies typically become clearly recognizable after about 48 hours because Y. enterocolitica grows more slowly than many other enterics, allowing the characteristic morphology to develop. Other organisms may grow on CIN but do not form this bull's-eye pattern within the same timeframe, making this appearance particularly diagnostic for Yersinia enterocolitica.