In-vitro activity of streptogramin RP 59500 against staphylococci, including bactericidal kinetic studies

The in-vitro activity of RP 59500 and comparative agents was determined against 270 clinical isolates of the genus Staphylococcus. MICs were performed by micro-dilution dilution. MIC90 and MBC90 (mg/L) of RP 59500 were as follows: oxacillin-sensitive Staphylococcus aureus (0.5/0.5), oxacillin-resistant S. aureus (0.5/0.5), oxacillin-sensitive Staphylococcus epidermidis (0.5/0.5), oxacillin-resistant S. epidermidis (0.25/0.25), oxacillin-resistant Staphylococcus hominis (1.0/1.0), oxacillin-resistant Staphylococcus haemolyticus (1.0/1.0), oxacillin-sensitive Staphylococcus saprophyticus (1.0/1.0). Killing kinetic methods were used to assess the bactericidal activity of inhibitory (1 x, 2 x MIC) concentrations of RP 59500 in comparison with that of vancomycin (1 x, 2 x MIC) and oxacillin (1 x, 2 x MIC) against 20 strains of oxacillin-sensitive and -resistant S. aureus and S. epidermidis. RP 59500 was as active as vancomycin, displaying rapid bactericidal activity against the majority of strains tested and reducing initial inoculum counts by greater than 99.9% in 2-12 h. Regrowth was seen with some S. epidermidis strains after 12-24 h.