Double zone hemolysis on anaerobic blood agar is characteristic of which organism?

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

Answer: (A)

Double zone hemolysis on anaerobic blood agar comes from a bacterium that releases a potent lecithinase toxin, which aggressively attacks red blood cell membranes. The dense growth of this organism concentrates the toxin in the center, producing a clear inner zone of complete hemolysis, while a surrounding, less intense area shows partial hemolysis. This two-zone pattern is a classic clue for Clostridium perfringens, an anaerobic Gram-positive, spore-forming rod associated with gas gangrene and food poisoning. The other organisms listed do not characteristically produce this distinctive dual-zone hemolysis on anaerobic blood agar—one is a non-hemolytic or differently hemolytic Clostridium species, and the others are anaerobic oral bacteria with different, non–double-zone patterns.