Outbreak of spontaneous staphylococcal arthritis and osteitis in mice. Staphylococcus aureus is the most common bacterial species found in association with nongonococcal bacterial arthritis in humans. We present here the first description of spontaneous bacterial arthritis and osteitis in mice. Clinically, the most obvious findings were swelling and/or ankylosis of hindpaws and nodose changes of the tail. The prevalence of arthritis and osteitis ranged from 0% to greater than 50% of the mice studied, depending on the mouse strain. The most prominent histopathologic feature of the arthritis was hypertrophy of the synovial tissue and destruction of cartilage and underlying bone. Most of the S aureus-infected mice displayed an identical phage type, which was also the only S aureus phage type found in skin isolates from clinically healthy mice. However, a few S aureus isolates were not typeable, indicating that an additional strain(s) might cause bacterial arthritis in mice. 