The bactericidal effects of anti-MRSA agents with rifampicin and sulfamethoxazole-trimethoprim against intracellular phagocytized MRSA

We experienced therapeutic failure with vancomycin in patients with serious methicillin-resistant Staphylococcus aureus (MRSA) infections, in some of whom the bacteria were found to be alive in the leukocytes. We therefore evaluated the antimicrobial activity of several anti-MRSA agents (vancomycin, linezolid, quinupristin/dalfopristin, arbekacin) and co-administered agents (rifampicin, sulfamethoxazole-trimethoprim) against clinically isolated MRSA in phagocytized human polymorphonuclear leukocytes. After allowing the leukocytes to phagocytize the bacteria, the mixture was separated into leukocytes and supernatant, to which MRSA agents were added, and incubated for 24 h. After incubation, the leukocytes were crushed and the intracellular MRSA was cultured quantitatively. Vancomycin resulted in a less than 1% survival ratio of extracellular MRSA, but it was one of the highest ratios of intracellular MRSA with 33.8% compared with other agents. The survival ratios of intracellular MRSA with vancomycin plus rifampicin and with vancomycin plus rifampicin plus sulfamethoxazole-trimethoprim were 0.78% and 1.02%, respectively, which is significantly lower than that of vancomycin. For linezolid, quinupristin/dalfopristin, and arbekacin, there were no significant differences in the survival ratios between monotherapy and combination therapy against either extracellular or intracellular MRSA. The results suggest that the concomitant use of rifampicin or rifampicin plus sulfamethoxazole/trimethoprim with vancomycin is effective for MRSA phagocytized in leukocytes when vancomycin monotherapy is not sufficiently effective. Combination therapy showed no difference in efficacy in the case of linezolid, quinupristin/dalfopristin, and arbekacin.