In-vitro selection of resistance of Staphylococcus aureus to teicoplanin and vancomycin.

Nineteen strains of Staphylococcus aureus (five penicillin-susceptible and methicillin-susceptible, four methicillin-susceptible and penicillin-resistant, and ten methicillin-resistant) were serially subcultured in broth media containing a subinhibitory concentration (half the MIC) of either teicoplanin or vancomycin. The MIC of the antibiotic was again measured after five passages in antibiotic-containing broth. The experiment was terminated after 25 passages. After passage fifteen strains exhibited an eight-fold or greater increase in MIC of teicoplanin. On the other hand, only two strains exhibited even an unstable four-fold increase in MIC of vancomycin.