Reversion of the glycopeptide resistance phenotype in Staphylococcus aureus clinical isolates

Abstract

The recent identification of glycopeptide intermediate-resistant Staphylococcus aureus (GISA) clinical isolates has provided an opportunity to assess the stability of the glycopeptide resistance phenotype by nonselective serial passage and to evaluate reversion-associated cell surface changes. Three GISA isolates from the United States (MIC of vancomycin = 8 microg/ml) and two from Japan (MICs of vancomycin = 8 and 2 microg/ml) were passaged daily on nutrient agar with or without vancomycin supplementation. After 15 days of passage on nonselective medium, vancomycin- and teicoplanin-susceptible revertants were obtained from each GISA isolate as determined by broth dilution MIC. Revertant isolates were compared with parent isolates for changes in vancomycin heteroresistance, capsule production, hemolysis phenotype, coagulase activity, and lysostaphin susceptibility. Several revertants lost the subpopulations with intermediate vancomycin resistance, whereas two revertants maintained them. Furthermore, although all of the parent GISA isolates produced capsule type 5 (CP5), all but one revertant tested no longer produced CP5. In contrast, passage on medium containing vancomycin yielded isolates that were still intermediately resistant to vancomycin, had no decrease in the MIC of teicoplanin, and produced detectable CP5. No consistent changes in the revertants in hemolysis phenotype, lysostaphin susceptibility, or coagulase activities were discerned. These data indicate that the vancomycin resistance phenotype is unstable in clinical GISA isolates. Reversion of the vancomycin resistance phenotype might explain the difficulty in isolating vancomycin-resistant clinical isolates from the blood of patients who fail vancomycin therapy and, possibly, may account for some of the difficulties in identifying GISA isolates in the clinical laboratory.

FIG. 1

Population analysis of parent GISA isolates MI, NJ, PC, and Mu50 and revertants. Strains grown overnight were serially diluted and plated on various concentrations of vancomycin-containing agar medium as described in Materials and Methods. “[P60]” refers to the revertant isolate obtained by passaging the MI parent strain for 60 days. “[P15]” refers to the revertant isolate obtained by passaging the indicated parent strain for 15 days. Points appearing on the abscissa refer to zero CFUs.