Influence of erythromycin resistance, inoculum growth phase, and incubation time on assessment of the bactericidal activity of RP 59500 (quinupristin-dalfopristin) against vancomycin-resistant Enterococcus faecium

Abstract

256 μg/ml) and 7 erythromycin-intermediate (MIC, 2 to 4 μg/ml) strains. All isolates were inhibited by RP 59500 at 0.25 to 1.0 μg/ml. The bactericidal activity of RP 59500 was markedly influenced by the erythromycin susceptibility of the strains and by several technical factors, such as inoculum growth phase and time of incubation of counting plates. As determined by time-kill methods, RP 59500 at a concentration of 2 or 8 μg/ml failed to kill erythromycin-resistant organisms under any conditions. Bactericidal activity was observed against all seven erythromycin- intermediate isolates when log-phase inocula were used and the cells were counted after 48 h of incubation (mean reductions in viable bacteria for RP 59500 at concentrations of 2 and 8 μg/ml, 3.45 and 3.50 log10 CFU/ml, respectively), but killing was diminished when the plates were examined at 72 h (mean killing, 3.06 and 2.95 log10 CFU/ml, respectively). No bactericidal activity was observed when stationary-phase cultures were used. On the basis of these date, we expect that bactericidal activity of RP 59500 against the multiple-drug-resistant E. faecium strains currently encountered would he distinctly uncommon.