Intra-abdominal fluid culture reveals few anaerobic, gram-negative, rod-shaped bacteria. Which organism is most likely responsible?

• A. Bacteroides fragilis
• B. Escherichia coli
• C. Klebsiella pneumoniae
• D. Bacteroides thetaiotaomicron

Answer: (A)

Intra-abdominal infections are often driven by anaerobic bacteria from the gut, and among anaerobic gram-negative rods, one organism stands out as the most frequently isolated in these settings: Bacteroides fragilis. This species thrives in low-oxygen environments like the peritoneal cavity and is a common cause of intra-abdominal abscesses and polymicrobial peritonitis. Its capsule helps it resist phagocytosis, contributing to persistence in sterile spaces and making it a classic culprit when anaerobes are identified on culture.

The other organisms listed are relevant in abdominal infections too, but they differ in a couple of key ways. Some are facultative anaerobes, meaning they can grow with or without oxygen, which makes them less.characteristic when the culture highlights strict anaerobes. Bacteroides thetaiotaomicron is an anaerobic gut bacterium, but it is less commonly the dominant isolate in intra-abdominal infections than fragilis. So, given the finding of an anaerobic, gram-negative, rod-shaped organism most typically associated with intra-abdominal infections, Bacteroides fragilis is the best choice.