An anaerobic blood agar plate inoculated with material from a penetrating farm injury shows, after 48 hours, a thin film of growth spreading over the entire surface of the plate, obscuring individual colonies. Smear from the edge of the film shows slender Gram-positive bacilli with round terminal spores. The organism is:

• A Clostridium tetani ✓
• B Clostridium perfringens
• C Clostridium botulinum
• D Clostridium difficile

Explanation

Clostridium tetani is strictly anaerobic, extremely motile, and swarms as a fine spreading film over the whole agar surface, a feature so troublesome that phenylmercuric nitrate or heating at 80 degrees Celsius is used to restrict it. The drumstick appearance from round terminal spores confirms the identity. Perfringens produces discrete colonies with double zone haemolysis and forms subterminal oval spores rarely seen in vitro.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.