A case of septic arthritis in multiple joints due to Bacteroides fragilis in a patient with rheumatoid arthritis.

A case of arthritis in multiple joints due to B. fragilis ssp. fragilis is described. The patient was a 55-year-old man badly disabled by rheumatoid arthritis. The patient's immune response to the isolated strain was studied and a significant titre reduction during the course of the disease was observed. The serum antibodies of the patient cross-reacted with heterologous strains of B. fragilis ssp. fragilis. It is suggested that indirect immunofluorescence tests against a certain number of B. fragilis strains could be performed with serum from a patient in whom an infection with B. fragilis is suspected.