Native Knee Septic Arthritis Due to Cutibacterium acnes: A Case Report.

Commensal skin anaerobes have been described as causative agents of prosthetic joint infections. Infection of native joints by these agents are, however, less common. We present the case of a 34-year-old male with recurrent joint effusion following closed trauma to the knee, four years ago, refractory to corticosteroid injections and several arthrocenteses. A synovial biopsy revealed Cutibacterium acnes infection leading to antibiotic therapy with clindamycin, and the patient was referred to orthopaedic and submitted to arthroscopic lavage. Atypical cartilage lesions, resembling the "growth of bacterial colonies", were found in the tibial plateaus with repeated isolation of C. acnes. Inpatient treatment with penicillin and vancomycin was conducted, followed by an oral course of amoxicillin, with no further registered recurrences. In this case, the authors describe a rare cause of native knee septic arthritis while highlighting the importance of repeated microbiology studies and adequate collection technique and sample handling, in order to better ascertain whether the isolated agent represents a contaminated sample or a true infection.