An anaerobically incubated blood agar plate shows colonies surrounded by an inner zone of complete red cell lysis and an outer zone of incomplete cell lysis (double zone of beta-hemolysis). The most likely presumptive identification of this isolate would be which organism?

• A. Clostridium perfringens
• B. Clostridium tetani
• C. Fusobacterium nucleatum
• D. Prevotella melaninogenica

Answer: (A)

Think first about how hemolysis patterns on blood agar help identify organisms. A distinctive double zone of beta-hemolysis—inner complete red-cell lysis with an outer zone of incomplete lysis—is a classic hallmark associated with Clostridium perfringens when grown under anaerobic conditions.

This pattern arises because the central colonies produce strong, potent effects (complete lysis) that erase the red cells directly around the bacteria, while the surrounding area shows only partial lysis due to diffusion of toxins or different activity outside the immediate colony. Among the organisms listed, Clostridium perfringens is the one most characteristically described as creating that two-zone beta-hemolysis on anaerobic blood agar. The others may show beta-hemolysis or hemolysis patterns, but not the distinct double-zone pattern described here, making they less likely matches.

So, the described colony with a central zone of complete lysis and an outer zone of incomplete lysis on an anaerobically incubated blood agar plate points most strongly to Clostridium perfringens.