Postantibiotic effect of clarithromycin alone and combined with ethambutol against Mycobacterium avium complex.

The postantibiotic effect (PAE) of clarithromycin alone and in combination with ethambutol was determined for two clinical blood isolates of Mycobacterium avium complex. An average PAE, ranging from 5.5 to 18.0 h, was noted for each isolate at each clarithromycin concentration except when isolate B was exposed to clarithromycin at the MIC. The addition of ethambutol did not enhance the PAE observed with clarithromycin alone. The clinical implications of the PAE of clarithromycin for M. avium complex remain to be determined.

Bactericidal activities of antibiotics against vancomycin-resistant Enterococcus faecium blood isolates and synergistic activities of combinations.

The effects of teicoplanin (8 micrograms/ml), ampicillin (64 micrograms/ml), imipenem (32 micrograms/ml), and gentamicin (4 micrograms/ml), alone and in combination, against 13 unique blood isolates of vancomycin-resistance (MIC for 90% of isolates tested [MIC90], 512 micrograms/ml), teicoplanin-susceptible (MIC90, 2.0 micrograms/ml), ampicillin-resistant (MIC90, 128 micrograms/ml), and non-beta-lactamase-producing Enterococcus facium (vancomycin-resistant enterococci [VRE] isolates) were evaluated by time-kill studies. All 13 isolates exhibited high-level resistance to streptomycin; 7 isolates exhibited high-level gentamicin resistance (HLGR). After 24 h of incubation, ampicillin (64 micrograms/ml) combined with gentamicin (4 micrograms/ml) was bactericidal against three of the VRE isolates that did not display HLGR. Synergy between ampicillin and gentamicin was not observed against these isolates. Teicoplanin (8 micrograms/ml) alone was bactericidal at 24 h against five of six VRE isolates that lacked HLGR, but was not bactericidal against any HLGR VRE isolate at that time point. The addition of ampicillin (64 micrograms/ml) or imipenem (32 micrograms/ml) to teicoplanin did not significantly enhance the killing of HLGR VRE isolates as a group (P = 0.335). However, there was a trend toward improved killing of some HLGR VRE isolates by teicoplanin plus imipenem. Vancomycin (32 micrograms/ml) combined with ampicillin (64 micrograms/ml) was neither bactericidal nor synergistic against HLGR VRE isolates. Overall, bactericidal activity was attainable against 7 of 13 VRE isolates at 24 h.