A 59-year-old male with fever, abdominal pain, and foul wound drainage after abdominal surgery; a deep tissue biopsy is cultured. BAP shows smooth gray colonies; Bacteroides esculin agar shows black colonies; CHOC is non-growth; Gram stain shows small Gram-negative bacilli. Which organism is suspected?

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The pattern points to an anaerobic, bile-esculin–positive gram-negative rod from a postoperative abdominal wound. The key clue is the black colonies on bile-esculin agar, which indicate esculin hydrolysis by the Bacteroides fragilis group. This organism is a common cause of intra‑abdominal and soft tissue infections after GI surgery. It grows as smooth gray colonies on blood agar under anaerobic conditions, and it typically does not grow on chocolate agar, matching the non-growth on CHOC in this scenario. The Gram stain shows small gram-negative bacilli, which fits Bacteroides. Among the options, this combination of clinical setting and lab features—especially the bile-esculin–positive, black BEA colonies—most specifically points to Bacteroides fragilis.