[Effect of trimethoprim at a subinhibitory concentration on the appearance of mutants resistant to rifampicin].

Experiments have been performed to establish the effect of trimethoprim (Tmp) on the emergence of Staphylococcus aureus (S. aureus) mutants to rifampicin (Ramp). Four S. aureus strains were used: two strains with low MIC of Ramp (0,01 microgram/ml), two strains with enhanced MIC of Ramp (2 micrograms/ml). For each of them the frequency of mutants and the mutation rates to Ramp were determined when various concentrations of Tmp were added. Results showed that subinhibitory concentrations of Tmp reduced 10 to 100 folds the frequency of mutants and the rates of mutation to Ramp alone. The more the Tmp concentrations were near the MIC, the more this effect was important.