Evidence from an In Vitro Study: Is Oxacillin Plus Vancomycin a Better Choice for Heteroresistant Vancomycin-Intermediate Staphylococcus aureus?

INTRODUCTION: Heteroresistant vancomycin-intermediate Staphylococcus aureus (hVISA) bacteremia may result in clinical failure of vancomycin therapy, together with prolonged infection and hospitalization. This clinical problem has resulted in a search for more effective treatment options. The current study was designed to further investigate the synergistic effect of oxacillin plus vancomycin against methicillin-resistant S. aureus (MRSA) and hVISA using checkerboard and time-kill assays. METHODS: Non-duplicate S. aureus isolates including hVISA (n = 29), MRSA (n = 10) and methicillin susceptible S. aureus (MSSA, n = 11) were used for combinational testing using checkerboard and time-kill assays. RESULTS: Twenty-one isolates, 15 hVISA and 6 MRSA, showed synergy between oxacillin and vancomycin by checkerboard assay with fractional inhibitory concentration indices of ≤ 0.5. The addition of oxacillin to vancomycin resulted in a reduction in baseline vancomycin MIC from 1–2 to 0.06–0.5 µg/ml against MRSA and hVISA isolates. In the time-kill assay, the combination of oxacillin and vancomycin resulted in synergistic activity against hVISA (n = 23) and MRSA (n = 7) isolates. Regrowth was observed in six hVISA isolates exposed to combination in the time-kill assay, but none of them reached the original inoculum density at 24 h. All re-growth isolates showed a onefold increase in vancomycin MIC (from 1 to 2 µg/ml) and were re-confirmed as hVISA using the population-analysis profile experiment. Overall, for hVISA and MRSA, the combination of oxacillin plus vancomycin had greater antibacterial effect than each individual drug alone. CONCLUSION: The present study showed the potential activity of vancomycin plus oxacillin combination against hVISA and MRSA isolates. Further, continued evaluation of this combination is warranted and may have therapeutic benefits in treating complicated MRSA infections.