In Vitro Pharmacodynamics of Vancomycin and Cefazolin Alone and in Combination against Methicillin-Resistant Staphylococcus aureus

Previous studies employing time-kill methods have observed synergistic effects against methicillin-resistant Staphylococcus aureus (MRSA) when a beta-lactam is combined with vancomycin. However, these time-kill studies have neglected the importance of human-simulated exposures. We evaluated the effect of human simulated exposures of vancomycin at 1 g every 8 h (q8h) in combination with cefazolin at 1 g q8h against various MRSA isolates. Four clinical isolates (two MRSA isolates [vancomycin MICs, 0.5 and 2.0 mu g/ml], a heterogeneous vancomycin-intermediate S. aureus [hVISA] isolate [MIC, 2.0 mu g/ml], and a vancomycin-intermediate S. aureus [VISA] isolate [MIC, 8.0 mu g/ml]) were evaluated in an in vitro pharmacodynamic model with a starting inoculum of 106 or 108 CFU/ml. Bacterial density was measured over 48 to 72 h. Time-kill curves were constructed, and the area under the bacterial killing and regrowth curve (AUBC) was calculated. During 106 CFU/ml studies, combination therapy achieved greater log(10) CFU/ml changes than vancomycin alone at 12 h (-4.31 +/- 0.58 versus -2.80 +/- 0.59, P < 0.001), but not at 48 h. Combination therapy significantly reduced the AUBC from 0 to 48 h (122 +/- 14) compared with vancomycin alone (148 +/- 22, P = 0.017). Similar results were observed during 10(8) CFU/ml studies, where combination therapy achieved greater log(10) CFU/ml changes at 12 h than vancomycin alone (-4.00 +/- 0.20 versus -1.10 +/- 0.04, P < 0.001) and significantly reduced the AUBC (275 +/- 30 versus 429 +/- 37, P < 0.001) after 72 h of incubation. In this study, the combination of vancomycin and cefazolin at human-simulated exposures improved the rate of kill against these MRSA isolates and resulted in greater overall antibacterial effect, but no differences in bacterial density were observed by the end of the experiments.