Evaluation of Vancomycin in Combination with Piperacillin-Tazobactam or Oxacillin against Clinical Methicillin-Resistant Staphylococcus aureus Isolates and Vancomycin-Intermediate S. aureus Isolates In Vitro

Vancomycin with piperacillin-tazobactam is used as empirical therapy for critically ill patients. Studies of this combination against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-intermediate S. aureus (VISA) are limited, but beta-lactams in combination with vancomycin have shown synergistic activity against MRSA and VISA. The goal of this study was to evaluate whether piperacillin-tazobactam and vancomycin were synergistic against MRSA and VISA in vitro. Bloodstream MRSA (n = 20) and VISA (n = 4) strains were selected. In vitro antimicrobial activities of piperacillin-tazobactam and oxacillin were evaluated by disk diffusion, and MICs were determined by Etest using Muller-Hinton agar with and without vancomycin at one-half the MIC. Time-kill studies evaluated 14 MRSA and all 4 VISA isolates using piperacillin-tazobactam at 300/35 mg/liter or oxacillin at 40 mg/liter alone and with vancomycin at one-half the MIC. Mean zones of inhibition for piperacillin-tazobactam and oxacillin increased with vancomycin against MRSA and VISA (P< 0.001 for all), and the MIC90 decreased with vancomycin against MRSA and VISA to values meeting susceptibility criteria for S. aureus (P< 0.001 for both antibiotics against MRSA). In MRSA time-kill studies, the mean 24-h reductions in inoculum for piperacillin-tazobactam, piperacillin-tazobactam with vancomycin, and oxacillin with vancomycin were 3.53, 3.69, and 2.62 log(10) CFU/ml, respectively. The mean 24-h reductions in VISA inoculum for piperacillin-tazobactam, piperacillin-tazobactam with vancomycin, and oxacillin with vancomycin were 2.85, 2.93, and 3.45 log(10) CFU/ml, respectively. Vancomycin with piperacillin-tazobactam or oxacillin demonstrated synergistic activity against MRSA and VISA. The clinical implications of these combinations against MRSA and VISA should be investigated.