Corynebacterium pyogenes septic arthritis with plasma cell synovial infiltrate and monoclonal gammopathy.

A chronic septic process developed in the right knee of an elderly man with advanced degenerative arthritis of both knees. Open exploration, culture, and biopsy of the joint found that the pathogen was Corynebacterium pyogenes and that the synovium was involved with a remarkable perivascular infiltrate of plasma cells. Serum protein electrophoresis demonstrated a prominent M component. Following antimicrobial therapy, the M protein level has gradually declined, and no evolution of multiple myeloma has become apparent. The findings are consistent with a benign monoclonal gammopathy and localized plasmacytic reaction in the knee associated with infection by an unusual diphtheroid organism.