A Case of Native Joint Septic Arthritis Caused by Bacteroides fragilis

BACKGROUND Bacteroides fragilis (B. fragilis) is an uncommon cause of native joint septic arthritis (NJSA). It is an anaerobic gram-negative rod that is normally found in the oral cavity, gastrointestinal tract, genitourinary system, and skin, and thus is rarely isolated from the musculoskeletal system. Risk factors for B. fragilis NJSA include inflammatory arthritis, malignancy, sickle cell disease, and ischemic heart disease. CASE REPORT We discuss a case of a 65-year-old man with coronary artery disease, heart failure, chronic kidney disease, and gout, who presented with 5 days of right knee pain, redness, swelling, and warmth. His history included a corticosteroid injection in the affected knee 2 months prior to presentation. The patient was febrile with a palpable right knee joint effusion on exam. Labs were significant for leukocytosis and elevated inflammatory markers. Synovial fluid was aspirated, which was diagnostic for septic arthritis, with cultures growing B. fragilis. Blood cultures were negative, so the infection was attributed to the prior joint injection. The patient was subsequently taken to the operating room for irrigation and debridement. He was started on metronidazole, but he unfortunately left against medical advice before treatment was complete and was lost to follow-up thereafter. CONCLUSIONS B. fragilis NJSA most often occurs in the setting of bacteremia or contiguous spread from a concomitant infection. Management involves antibiotics such as metronidazole and surgical drainage. Due to the paucity of data on an infection such as in the present case, the optimal duration of metronidazole is not well-established.