Effect of combination of oxacillin and non-beta-lactam antibiotics on methicillin-resistant Staphylococcus aureus.

The in-vitro activity of oxacillin combined with non-beta-lactam antibiotics, bacitracin, vancomycin, enduracidin, tunicamycin, flavomycin, fosfomycin, and cycloserine, was investigated in 23 methicillin-resistant and 15 methicillin-susceptible Staphylococcus aureus strains. In the presence of a non-beta-lactam antibiotic (0.25 MIC), the MICs of oxacillin for methicillin-resistant S. aureus (MRSA) strains and methicillin-susceptible S. aureus (MSSA) strains were lowered. This effect was most marked with MRSA strains, and bacitracin, flavomycin, and tunicamycin increased the susceptibility of MRSA to oxacillin by greater than 200-fold. More than 87% and 77% of MRSA and MSSA strains, respectively, were synergically inhibited by a combination of oxacillin with bacitracin, tunicamycin, flavomycin or cycloserine (fractional inhibitory concentration < or = 0.5). Polyanetholesulfonic acid prevented the lysis of MRSA cells treated with a combination of oxacillin and 0.25 MIC of bacitracin, but did not prevent cell death. The penicillin binding protein (PBP) profile of MRSA was not affected by incubation with 0.25 MIC of non-beta-lactam antibiotics. These results suggest that the increased antibacterial activity of oxacillin in the presence of non-beta-lactam antibiotic is not the consequence of activation of autolysis or of a decrease in bulk PBP synthesis.