An obligately anaerobic gram-negative bacillus isolated from an abdominal wound is resistant to penicillin and grows in the presence of bile. What is the most likely identification?

• A. Clostridium septicum
• B. Fusobacterium nucleatum
• C. Bacteroides fragilis group
• D. Eubacterium lentum

Answer: (C)

When an obligate anaerobic gram-negative bacillus from an abdominal wound is resistant to penicillin and can grow in the presence of bile, the organism most likely is the Bacteroides fragilis group. These bacteria are the dominant anaerobes in intra-abdominal infections and commonly produce beta-lactamase, making them penicillin-resistant. They also tolerate bile, which helps distinguish them from other anaerobic gram-negative rods. The other options are not a match: Clostridium septicum is gram-positive and spore-forming, Fusobacterium nucleatum is typically associated with the oral cavity and not characteristically penicillin-resistant or bile-tolerant to this degree, and Eubacterium lentum (Eggerthella lenta) is gram-positive.