SIMPLE TEST OF SYNERGY BETWEEN AMPICILLIN AND VANCOMYCIN FOR RESISTANT STRAINS OF ENTEROCOCCUS-FAECIUM

The combination of ampicillin and vancomycin kills some but not all strains of ampicillin- and vancomycin-resistant Enterococcus faecium. We compared a simple test for synergy utilizing a commercially available microdilution susceptibility system with time-kill studies and determined acceptable breakpoints for this test for 20 strains of ampicillin and vancomycin-resistant E. faecium. The combination of ampicillin and vancomycin was tested for synergy by time-kill, broth macrodilution, and broth microdilution procedures. Repeat testing of isolates by macro- and microdilution synergy methods yielded MICs that were,within one twofold dilution of each other for both intra- and intertest comparisons. Synergy was always detected by time kill studies when the MIC of ampicillin in the combination microdilution synergy screen was less than or equal to 8 mu g/ml in the presence of vancomycin. No synergy was detected when the MIC was >16 mu g/ml in the combination microdilution synergy screen. The determination of the synergy by the broth microdilution procedure appears to be simple, convenient, and accurate.