Which organism is classically associated with boxcar Gram-positive rods and double-zone hemolysis on blood agar?

• A. Streptococcus agalactiae
• B. Listeria monocytogenes
• C. Staphylococcus aureus
• D. Clostridium perfringens

Answer: (D)

Boxcar-shaped Gram-positive rods that form spores and grow as anaerobes point to Clostridium perfringens. A key lab clue is the distinct double-zone hemolysis on blood agar: an inner area of complete (beta) hemolysis right around the colony, with an outer surrounding zone of partial (alpha) hemolysis, produced by its potent toxins. This combination—rod shape, anaerobic spore-forming lifestyle, and the two-zone hemolysis pattern—is classically associated with C. perfringens and helps differentiate it from organisms like cocci or non–boxcar-shaped rods.