Adaptation of methicillin-resistant Staphylococcus aureus in the face of vancomycin therapy.

For the past 2 decades, vancomycin has served as the cornerstone of therapy against serious methicillin-resistant Staphylococcus aureus infections. This role is increasingly challenged by questions of efficacy, including reduced efficacy against infections caused by glycopeptide-intermediate S. aureus strains. In an evaluation of clinical glycopeptide-intermediate S. aureus isolates and serial, clinical methicillin-resistant S. aureus isolates obtained from patients receiving vancomycin for the treatment of bacteremia, we found that loss of function of the accessory gene regulator operon may confer a survival advantage to S. aureus under vancomycin selection pressure, particularly in strains with the accessory gene regulator group II genotype. Other advantages in a nosocomial setting may include enhancement of biofilm formation and promotion of physiologic changes supporting colonization. We conclude that loss of accessory gene regulator function in methicillin-resistant S. aureus might, in part, explain the decreased efficacy of vancomycin in the therapy of methicillin-resistant S. aureus bacteremia, thus highlighting the need to reevaluate the criteria of susceptibility to vancomycin.