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 st rains 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 determ ined acceptable breakpoints for this test for 20 strains of ampicillin and vancomycin-resistant E. faecium. The combination of ampicillin an d vancomycin was tested for synergy by time-kill, broth macrodilution, and broth microdilution procedures. Repeat testing of isolates by mac ro- and microdilution synergy methods yielded MICs that were,within on e twofold dilution of each other for both intra- and intertest compari sons. Synergy was always detected by time kill studies when the MIC of ampicillin in the combination microdilution synergy screen was less t han or equal to 8 mu g/ml in the presence of vancomycin. No synergy wa s detected when the MIC was >16 mu g/ml in the combination microdiluti on synergy screen. The determination of the synergy by the broth micro dilution procedure appears to be simple, convenient, and accurate.