Retrospective screening for heterogeneous vancomycin resistance in diverse Staphylococcus aureus clones disseminated in French hospitals

Twenty-five different Staphylococcus aureus strains that are widespread in France were screened by various methods for heterogeneous and low-level resistance to vancomycin. Population analysis on brain-heart infusion agar containing 4 mg/L of the drug detected resistant cells at frequencies of 10(-7) to 10(-6) in five multiply resistant strains. There was no antagonism between vancomycin and beta-lactam antibiotics. One of the five strains, isolated in 1993, was considered a putative progenitor of a French nosocomial S. aureus strain isolated in 1998 and for which the vancomycin MIC was 8 mg/L.