Septic arthritis due to Corynebacterium propinquum: first report of isolation from synovial fluid in a native joint

Septic arthritis of native joints remains an uncommon but serious clinical condition affecting approximately two per 100,000 individuals annually. The synovial membrane, being highly vascular and lacking a protective basement membrane, is particularly susceptible to hematogenous bacterial seeding, especially in patients with pre-existing joint disease. Corynebacterium propinquum, typically regarded as a respiratory commensal, has rarely been implicated in deep-seated infections.

We report the first known case of C. propinquum isolated from synovial fluid in a patient with native knee septic arthritis. The 59-year-old female had a history of bilateral knee osteoarthritis and recent intra-articular corticosteroid injections. Initial synovial fluid cultures were negative; however, enrichment using aerobic blood culture bottles, followed by subculture, yielded C. propinquum identified by MALDI-TOF MS. The organism grew on aerobic blood agar but not on anaerobic media or thioglycolate broth. The patient was successfully treated with intravenous daptomycin, followed by ceftriaxone, with subsequent clinical improvement.

This case highlights the diagnostic value of using enrichment culture and comprehensive laboratory-clinical correlation to recognize clinically significant, low-virulence organisms, such as Corynebacterium propinquum. Accurate species-level identification and thoughtful interpretation within the clinical context are essential to distinguish contamination from true infection, particularly in sterile-site cultures associated with native joint infections.