RexAB Is Essential for the Mutagenic Repair of Staphylococcus aureus DNA Damage Caused by Co-trimoxazole

Co-trimoxazole (SXT) is a combination therapeutic that consists of sulfamethoxazole and trimethoprim that is increasingly used to treat skin and soft tissue infections caused by methicillin-resistant Staphylococcus aureus (MRSA). However, the use of SXT is limited to the treatment of low-burden, superficial S. aureus infections and its therapeutic value is compromised by the frequent emergence of resistance. As a first step toward the identification of approaches to enhance the efficacy of SXT, we examined the role of bacterial DNA repair in antibiotic susceptibility and mutagenesis. We found that mutants lacking the DNA repair complex RexAB had a modest SXT MIC that was 2-fold lower than that seen with wild-type strains but were killed 50-fold to 5,000-fold more efficiently by the combination antibiotic at the breakpoint concentration. SXT-mediated DNA damage occurred via both thymidine limitation and the generation of reactive oxygen species and triggered induction of the SOS response in a RexAB-dependent manner. SOS induction was associated with a 50% increase in the mutation rate, which may contribute to emergence of resistant strains during SXT therapy. In summary, this work determined that SXT caused DNA damage in S. aureus via both thymidine limitation and oxidative stress and that the damage was repaired by the RexAB complex, leading to induction of the mutagenic SOS response. Small-molecule inhibitors of RexAB could therefore have therapeutic value by increasing the efficacy of SXT and decreasing the emergence of drug resistance during treatment of infections caused by S. aureus.