In-vitro effects of vancomycin and rifampicin, alone and in combination, against methicillin-sensitive and methicillin-resistant staphylococci.

The in-vitro inhibitory and bactericidal activities of vancomycin and rifampicin were tested against several Staphylococcus strains recently isolated from clinical material. All strains were first tested for their susceptibility to methicillin, and typed by lyogroup. With vancomycin, minimum inhibitory concentrations (MICs) fell within a narrow range (0.62-2.5 mg/l), and significant differences were not observed both among different lyogroups and between methicillin-sensitive (M-S) and methicillin-resistant (M-R) strains. Minimum bactericidal concentrations (MBCs) of vancomycin either equalled or exceeded by twofold the MICs. With rifampicin, MICs were found to be distributed over a wide range (0.002 to greater than 5 mg/l), and susceptibility appeared to be less in M-R than in M-S isolates. Although MICs greater than or equal to mg/l were only observed in three of the six lyogroups, data appeared insufficient to substantiate definite interspecies differences in staphylococcal susceptibility to rifampicin. MBCs of rifampicin either equalled or exceeded by two- to eightfold the MICs. In-vitro interactions between vancomycin and rifampicin, were investigated both by checkerboard and time-kill tests. By both methods, indifference was observed with the great majority of isolates. Synergism was a rare event, and antagonism was never observed.