Differences in ability of cell-wall antibiotics to suppress emergence of rifampicin resistance in Staphylococcus aureus.

Rifampicin resistance developed easily in methicillin-susceptible and methicillin-resistant strains of Staphylococcus aureus during an overnight incubation in broth containing 0.1 mg/l of rifampicin. Incubation of methicillin-susceptible Staph. aureus and 0.1 mg/l of rifampicin with 1 mg/l of nafcillin reduced the emergence of rifampicin resistance with only 5 of 50 strains (10%) becoming rifampicin-resistant. However, incubation of the methicillin-susceptible or methicillin-resistant strains with 0.1 mg/l of rifampicin and 1 mg/l of vancomycin did not prevent the development of rifampicin resistance. Rifampicin resistance developed in 25 of 50 (50%) of methicillin-susceptible and 32 of 50 (64%) methicillin-resistant Staph. aureus strains tested. These data would suggest that differences exist in the abilities of nafcillin and vancomycin to suppress the development of rifampicin resistance in Staph. aureus (P less than 0.01). Caution should be exercised when the combination of vancomycin and rifampicin is used for infections caused by Staph. aureus and Staph. aureus isolates recovered during therapy should be monitored for the development of rifampicin resistance.