DEVELOPMENT OF A STANDARDIZED SCREENING METHOD FOR DETECTION OF VANCOMYCIN-RESISTANT ENTEROCOCCI

The incidence of vancomycin resistance among enterococci is increasing in the United States and elsewhere in the world, but automated suscep tibility testing methods have difficulty detecting resistance expresse d by certain strains. The agar screening method described by Willey et al. (B. M. Willey, B. N. Kreiswirth, A. E. Simor, G. Williams, S. R. Scriver, A. Phillips, and D. E. Low, J. Clin. Microbiol. 30:1621-1624, 1992) has been proposed as a reliable method for confirming vancomyci n resistance. In this study, we investigated various parameters associ ated with the agar screening method and, on the basis of the findings, established optimum testing conditions for the method. First, to eval uate media and vancomycin concentrations, one laboratory used Mueller- Hinton and brain heart infusion agars supplemented with 4, 6, and 8 mu g of vancomycin per ml to test 100 genetically characterized enteroco ccal strains. On the basis of the results obtained, brain heart infusi on agar supplemented with 6 mu g of vancomycin per ml was selected for further study. Subsequently, eight laboratories used the medium to te st both reference and clinical isolates. There was very good performan ce with the reference strains and, among 158 clinical isolates tested, the method demonstrated sensitivity and specificity of 100% and from 96 to 99%, respectively.