Differential effect of rpoB mutations on antibacterial activities of rifampicin and KRM-1648 against Staphylococcus aureus

The in vitro antibacterial activities of the rifamycin derivatives rifampicin and KRM-1648 against 150 Staphylococcus aureus isolates were determined. The MICs of rifampicin and KRM-1648 for 90% of rifampicin-susceptible S. aureus isolates (n = 100) were 0.016 and 0.001 mg/L, respectively. In rifampicin-resistant S. aureus isolates (n = 50), different levels of resistance to rifamycins were associated with mutations at different sites in rpoB. Mutations at some sites were associated with high-level resistance to both rifamycins, while certain mutations were associated with the activity of KRM-1648 being < or = 100-fold better than that of rifampicin.