Unstable Vancomycin Heteroresistance Is Common among Clinical Isolates of Methiciliin-Resistant <i>Staphylococcus aureus</i>

We tested 109 unique, vancomycin-susceptible, methicillin-resistant Staphylococcus aureus (MRSA) strains for vancomycin heteroresistance by a selection method, i.e., step-wise exposure of large inoculums to increasing concentrations of vancomycin. Although no strains demonstrated stable heteroresistance, 81 strains (74%) demonstrated unstable heteroresistance. Unstable heteroresistance is common among clinical isolates of MRSA and may represent a cause of therapeutic failure.