Evidence that patients with rheumatoid arthritis have asymptomatic 'non-significant' Proteus mirabilis bacteriuria more frequently than healthy controls

Objectives: patients with rheumatoid arthritis (RA) are reported to have in their sera raised levels of antibody specific to Proteus mirabilis. The ai m of the study was to verify this and to determine an explanation for it by investigating the frequency of P. mirabilis urinary tract infection in RA patients and matched controls. Methods: freshly voided urine was examined for the presence, number and ide ntity of infecting bacteria. The levels of antibody in blood and in urine o f the IgM, IgA and IgG classes to the common O serotypes of P. mirabilis an d the antigens to which they reacted were determined by enzyme-linked immun osorbent assay (ELISA) and immunoblotting. Results: analysis of urine from 76 patients with RA and 48 age- and gender- matched healthy controls showed that only two (4%) of the control urines bu t 25 (33%) of those fi om the RA patients were infected. The commonest infe cting organism in the RA patients' urine was Proteus mirabilis which occurr ed twice as frequently as Escherichia coli. Proteus mirabilis was found in 52% of the infected urines of the RA patients and was always detected as a pure growth and usually in insignificant (< 10(4)/ml) numbers. It is highly improbable that this finding was the outcome of differences in age, physic al ability or medication between the RA and control patient groups. Compari son of antibody levels to P. mirabilis by ELISA showed RA patients had rais ed (P < 0.0001, P = 0.001, P = 0.0063) levels of IgA, IgG and IgM respectiv ely in their sera and raised (P < 0.0001, P < 0.0001, P = 0.0001) levels of IgG, IgM and IgA respectively in their urine compared with the control gro up. It was not possible to detect an antibody reacting to a P. mirabilis an tigen that was specific to the RA patients. Conclusion: the results confirm that RA patients have raised levels of anti body to P. mirabilis not only in blood but also in urine and suggest that t his arises because RA patients have an asymptomatic, non-significant P. mir abilis bacteriuria more frequently or more prolonged than control patients. This may be the trigger for their RA condition.