The continuous presence of bacteria or their degraded antigens in the synovium may be involved in the pathogenesis of rheumatoid arthritis (RA). The aim of this study was to determine the presence of bacterial nucleic acids and bacterial cell wall constituents in the joints of patients with RA and other forms of arthritis.

Joint samples were obtained from patients with RA (n = 26), septic arthritis (n = 2), inflammatory osteoarthritis (n = 5), and gout (n = 6), and joint trauma (n = 1). Universal 16S‐ribosomal RNA primers were used to detect the presence of bacterial DNA in these samples, using stringent regimens for sample collection and molecular microbiologic analysis. Automated sequencing and comparative data analysis were performed to identify the species. The presence of bacterial peptidoglycan–polysaccharide complexes in synovial tissue was detected by immunohistologic analysis with a specific antibody.

The bacterial species cultured from the synovium could be identified in both of the patients with septic arthritis. DNA amplicons were also detected in the synovial fluid and/or tissue samples from 5 patients with RA and 2 patients with crystal‐induced arthritis; these originated from multiple bacterial species. Staining for peptidoglycan–polysaccharide complexes was positive in the synovial tissue of both patients with septic arthritis, 16 with RA, 4 with inflammatory osteoarthritis, 4 with crystal‐induced arthropathy, and 1 with joint trauma. The staining was mainly found in cells in the synovial sublining, including macrophages.

The results indicate that bacterial DNA and bacterial cell wall constituents are retained in the joints of some patients with arthritis, where they might enhance synovial inflammation.