Effects of antibiotic combinations on methicillin-resistant Staphylococcus aureus in vitro.

Methicillin-resistant Staphylococcus aureus (MRSA) infection has become an important infectious disease problem. Vancomycin is the only antimicrobial agent documented to be effective in the treatment of MRSA infection. Under vancomycin treatment there is still a moderate percentage of treatment failures, especially in severe MRSA infections. In searching for better treatment regimens, we evaluated the effects of several antibiotics in combination with vancomycin or fosfomycin against MRSA isolates by the checkerboard titration method. We found that the combinations of vancomycin with imipenem and fosfomycin with cefazolin had a synergistic effect against 97.1% and 94.3% of 70 tested strains, respectively. Rifampicin, gentamicin, cefazolin, or cefamandole in combination with vancomycin, and cefmetazole in combination with fosfomycin did not have such good results. They were synergistic against far fewer of the tested strains, 0%, 3.3%, 50.0%, 30.0% and 64.3%, respectively. For confirmation, further studies in vivo of the effectiveness are advisable. It is likely that we can improve the treatment of severe MRSA infections by those combination regimens which are highly synergistic against MRSA.