In Vitro Activities of Co-Amoxiclav at Concentrations Achieved in Human Serum against the Resistant Subpopulation of HeteroresistantStaphylococcus aureus: a Controlled Study with Vancomycin

The effects of concentrations that simulated those in human serum after a single intravenous dose of amoxicillin (2 g), amoxicillin-clavulanic acid (2,000 and 200 mg, respectively), or vancomycin (500 mg), on the viability and beta-lactamase activity of two isogenic (beta-lactamase and non-beta-lactamase producer) heteroresistant Staphylococcus aureus strains were studied in an in vitro pharmacodynamic model. A reduction of > or = 97% of the initial inoculum was obtained with vancomycin and amoxicillin-clavulanic acid against both strains, with respect to the total bacterial population and the oxacillin-resistant subpopulation. The same pattern was observed with amoxicillin and the beta-lactamase-negative strain. beta-Lactamase activity in the beta-lactamase-positive strain changed over time parallel to viability, decreasing with amoxicillin-clavulanic acid or vancomycin and increasing in the amoxicillin and control groups. Clavulanic acid concentrations achievable in serum that changed over time allowed amoxicillin to act against the beta-lactamase-producing methicillin-resistant S. aureus to a similar extent as vancomycin.