Post-antibiotic effects of RP 59500 with Staphylococcus aureus

The post-antibiotic effect (PAE) of the semisynthetic streptogramin RP 59500 was evaluated with four clinical isolates of Staphylococcus aureus (two strains susceptible to methicillin and to the macrolide-lincosamide-streptogramin B (MLSB) group and two strains resistant to these drugs). The PAE was defined as the time required for either the viable counts to increase by one log10 or for bacteria to regain their maximal rate of growth. Similar results were obtained regardless of which definition was used. For each experiment, the time of exposure of the bacteria to the drug ranged from 15-80 min. At a concentration of 0.5 x MIC and an exposure time of 80 min, RP 59500 produced a PAE in three of the four strains examined. At higher concentrations (1, 2 and 4 x MIC), a PAE was observed with all four strains. When considered as the time required for bacterial growth to return to its maximal rate, the PAE lasted for at least 7 h with the two methicillin-susceptible strains when they were exposed for 80 min to RP 59500 at 4 x MIC, compared with 5 h for the two methicillin-resistant strains and the same exposure conditions. The concentration of antibiotic was found to be a more important parameter in determining the PAE than the time of exposure.