Increased rifampicin resistance in blood isolates of meticillin-resistant Staphylococcus aureus (MRSA) amongst patients exposed to rifampicin-containing antituberculous treatment

The aim of this study was to determine the rifampicin (RIF) resistance rate of meticillin-resistant Staphylococcus aureus (MRSA) amongst patients with MRSA bacteraemia who have or have not been exposed to RIF-containing antituberculous (anti-TB) treatment. From 2000 to 2008, patients with MRSA bacteraemia and previous exposure to RIF-containing anti-TB therapy were selected. Patients matched for sex, age and time of culture of MRSA bacteraemia but without exposure to anti-TB therapy were selected as a control group. A total of 139 patients, comprising 49 with RIF exposure and 90 without RIF exposure, were analysed. The RIF resistance rate was higher in patients with previous RIF exposure (61.2% vs. 20.0%; P<0.001). The minimum inhibitory concentration of RIF that inhibited 50% of MRSA isolates (MIC(50)) for the study group was also higher (128 mg/L vs. 0.015 mg/L; P<0.001). The mortality rate was higher in the study group (59.2% vs. 41.1%; P=0.041). MRSA isolates recovered from patients with current usage of a RIF-containing anti-TB regimen were more likely to be resistant to RIF (87.5% vs. 36%; P=0.001), with higher MIC(50) values (256 mg/L vs. 1mg/L; P=0.002), and resulted in a higher mortality rate than isolates from patients with remote usage of an anti-TB regimen (79.2% vs. 40%; P=0.005). Multivariate analysis showed that current anti-TB drug usage was the only risk factor for RIF resistance odds ratio (OR)=7.457, 95% confidence interval (CI) 1.581-35.167] and mortality (OR=7.201, 95% CI 1.583-32.766). Given the high rate of RIF resistance in patients with prior anti-TB treatment, RIF susceptibility testing should be performed before considering combination treatment of RIF in MRSA infection.