In-vitro induction of resistance in coagulase-negative staphylococci to vancomycin and teicoplanin.

Twenty strains of coagulase-negative staphylococci (18 strains of Staphylococcus haemolyticus and two of S. epidermidis) were serially subcultured in broth media containing subinhibitory concentrations (half the MIC) of either vancomycin or teicoplanin. The MIC of the antibiotic was again measured after five passages in antibiotic-containing broth. The organisms were then subcultured in broth containing antibiotic concentrations half of the new MIC value. The experiment was terminated after 25 passages. Only one strain developed a four-fold increase in vancomycin MIC. On the other hand, 16 strains developed four-fold MIC increases to teicoplanin.