Combination Effect of Vancomycin and β-Lactams against a Staphylococcus aureus Strain, Mu3, with Heterogeneous Resistance to Vancomycin

We tested the combined activity of vancomycin and seven β-lactam antibiotics against Staphylococcus aureus clinical strain Mu3, which displays heterogeneous resistance to vancomycin. When combined with vancomycin, four of the seven tested β-lactams exhibited an additive effect at or near their MICs, while all showed an antagonistic effect at lower, sub-MIC levels. This study implicated the unpredictable nature of combination therapy of β-lactams and vancomycin against S. aureus with reduced susceptibility to vancomycin.