False synergy between vancomycin and β-lactams against glycopeptide-intermediate Staphylococcus aureus (GISA) caused by inappropriate testing methods

Abstract

The combination of vancomycin and β-lactams is often considered synergistic and has been recommended for the treatment of glycopeptide-intermediate Staphylococcus aureus (GISA) infections. In this study, the combination of vancomycin or teicoplanin with different β-lactams was tested. When using NaCl 4% w/v, for better expression of heterogeneous resistance to β-lactams, with a longer (48-h) incubation period and a higher (107 CFU/mL) inoculum, the association of vancomycin with β-lactams was antagonistic. However, a synergistic effect was observed for teicoplanin under the same conditions. © 2004 Copyright by the European Society of Clinical Microbiology and Infectious Diseases.