Which organism is most commonly associated with double-zone beta-hemolysis on blood agar?

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

Answer: (A)

Double-zone beta-hemolysis on blood agar is a distinctive pattern most closely tied to Clostridium perfringens. The inner zone of complete hemolysis is produced by potent hemolysins like alpha-toxin (lecithinase) released by the organism. Surrounding this is a broader outer zone where hemolysis is still present but less intense, created by additional hemolysins diffusing into the surrounding agar. This two-ring appearance around the colonies is characteristic and helps distinguish C. perfringens from other anaerobic bacteria, which may be beta-hemolytic but do not typically show this paired, concentric pattern. The other organisms listed do not exhibit this classic double-zone pattern on blood agar, so they’re not as closely associated with it.