BACTERICIDAL ACTIVITIES OF ANTIBIOTICS AGAINST VANCOMYCIN-RESISTANT ENTEROCOCCUS-FAECIUM BLOOD ISOLATES AND SYNERGISTIC ACTIVITIES OF COMBINATIONS

The effects of teicoplanin (8 mu g/ml), ampicillin (64 mu g/ml), imipe nem (32 mu g/ml), and gentamicin (4 mu g/ml), alone and in combination , against 13 unique blood isolates of vancomycin-resistant (MIC for 90 % of isolates tested [MIC(90)], 512 mu g/ml), teicoplanin-susceptible (MIC(90), 2.0 mu g/ml), ampicillin-resistant (MIC(90), 128 mu g/ml), a nd non-beta-lactamase-producing Enterococcus faecium (vancomycin-resis tant enterococci [VRE] isolates) were evaluated by time-kill studies. All 13 isolates exhibited high-level resistance to streptomycin; 7 iso lates exhibited high-level gentamicin resistance (HLGR). After 24 h of incubation, ampicillin (64 mu g/ml) combined with gentamicin (4 mu g/ ml) was bactericidal against three of the VRE isolates that did not di splay HLGR. Synergy between ampicillin and gentamicin was not observed against these isolates. Teicoplanin (8 mu g/ml) alone was bactericida l at 24 h against five of six VRE isolates that lacked HLGR, but was n ot bactericidal against any HLGR VRE isolate at that time point. The a ddition of ampicillin (64 mu g/ml) or imipenem (32 mu g/ml) to teicopl anin did not significantly enhance the killing of HLGR VRE isolates as a group (P = 0.335). However, there was a trend toward improved killi ng of some HLGR VRE isolates by teicoplanin plus imipenem. Vancomycin (32 mu g/ml) combined with ampicillin (64 mu g/ml) was neither bacteri cidal nor synergistic against HLGR VRE isolates. Overall, bactericidal activity was attainable against 7 of 13 VRE isolates at 24 h.