Heterogeneous vancomycin resistance in Staphylococcus aureus does not predict development of vancomycin resistance upon vancomycin pressure.

Background: It is unclear whether Staphylococcus aureus with heterogeneous intermediate vancomycin resistance (hVISA) can develop vancomycin resistance faster than vancomycin-susceptible S. aureus (VSSA) strains.

Methods: We compared the kinetics of vancomycin MIC increase for 15 days of sustained in vitro vancomycin exposure for clinical hVISA (n = 12) and VSSA (n = 24) isolates, as well as for reference strains Mu3 (hVISA) and ATCC 29213 (VSSA). Clinical isolates were categorized as hVISA using the population analysis profile method. MICs were monitored for 15 days and the rate of MIC increase under exposure, for each strain, was evaluated in a linear regression model relative to time.

Results: All isolates acquired vancomycin resistance upon exposure. Vancomycin MICs increased faster for VSSA compared with hVISA isolates (P < 0.01).

Conclusions: The hVISA phenotype does not correspond to an enhanced adaptation potential to in vitro vancomycin pressure.