In vitro activity of quinupristin/dalfopristin against erythromycin-susceptible and erythromycin-resistant Streptococcus pneumoniae.

Minimal inhibitory concentrations (MICs) of quinupristin/dalfopristin, penicillin, erythromycin, and clindamycin were determined by a standard agar dilution method for 93 Streptococcus pneumoniae strains isolated from patients with invasive disease in Germany. Quinupristin/dalfopristin showed good activity against 32 penicillin-susceptible/erythromycin-susceptible strains (MIC90 0.5 mg/l; range 0.25-0.5 mg/l) and 31 penicillin-intermediate/erythromycin-susceptible strains (MIC90 0.5 mg/l; range 0.25-1 mg/l). Erythromycin-resistant strains (n=30) were slightly less susceptible (MIC90 1 mg/l; range 0.125-2 mg/l). Quinupristin/dalfopristin was bactericidal (99.9% killing) for all six strains investigated after 2 h at a concentration of 4 mg/l.