In-vitro selection of resistance to vancomycin and teicoplanin in Enterococcus faecium and Enterococcus faecalis compared with Staphylococcus epidermidis.

Clinical isolates of Enterococcus faecium, Enterococcus faecalis and Staphylococcus epidermidis were studied for their ability to develop resistance by two selection methods, one in broth and one on agar. The MICs for enterococci after exposure to vancomycin ranged from 0.5 mg/L to 4 mg/L, and those after exposure to teicoplanin ranged from 0.25 mg/L to 1 mg/L. No significant increases occurred for E. faecalis, whilst two isolates of E. faecium showed increases of up to eight-fold (from 0.125 mg/L to 1 mg/L). Vancomycin MICs ranged from 1 mg/L to 4 mg/L and teicoplanin MICs reached 32 mg/L for S. epidermidis. Pulsed-field gel electrophoresis analysis of SmaI-digested whole chromosomal DNA was performed to compare the genomic DNA of glycopeptide-exposed and wild-type strains. In-vitro exposure to vancomycin did not alter MICs significantly except in one S. epidermidis isolate for which the MIC reached 4 mg/L, whereas the teicoplanin-exposed cultures of S. epidermidis showed increases of up to 64 times the original MIC. Comparable results were achieved with the two selection methods.