Obligate anaerobic, gram-negative bacilli recovered from an abdominal wound showed resistance to penicillin and growth not inhibited by bile. Which organism is most likely?

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

Answer: (A)

Bacteroides fragilis is the best fit because it is an obligate anaerobic Gram-negative bacillus that is commonly recovered from abdominal infections and often resists penicillin due to beta-lactamase production. Its ability to grow in bile (bile not inhibiting its growth) helps differentiate it from other anaerobic Gram-negative rods that are inhibited by bile. Intra-abdominal infections are frequently caused by Bacteroides fragilis, making it the most likely organism in this scenario. The other organisms listed are either Gram-positive anaerobes or, while they are anaerobic, do not match the combination of penicillin resistance and bile-insensitive growth typical of the fragilis group.