Pyomyositis associated with Bacteroides fragilis in a patient with multiple myeloma.

Pyomyositis is an infection of the skeletal muscle that is usually caused by Staphylococcus aureus. We report a 68-year-old Japanese woman who developed pyomyositis caused by Bacteroides fragilis following treatment for multiple myeloma. There are only two cases of pyomyositis associated with multiple myeloma in the literature. After receiving melphalan and prednisolone for five days, she developed multiple abscesses in the muscles of the right arm and thigh. Purulent exudate was aspirated from the abscess, and B. fragilis was identified. This is the first case of B. fragilis pyomyositis. Magnetic resonance imaging aided the diagnosis. Treatment consisted of surgical incision and drainage, with antibiotic administration. The immunosuppression caused by the myeloma and/or the chemotherapy presented a risk factor for the unusual infection observed in this patient.