In vivo emergence of subpopulations expressing teicoplanin or vancomycin resistance phenotypes in a glycopeptide-susceptible, methicillin-resistant strain of Staphylococcus aureus

Several reports indicate the emergence of subpopulations resistant to glyco peptides in some clinical isolates of Staphylococcus aureus. While the deve lopment of glycopeptide resistance in S. aureus is easily observed in vitro , the in vivo conditions promoting emergence of glycopeptide-resistant subp opulations are unknown. Using a rat model, subcutaneous implants were chron ically infected with a methicillin-resistant strain of S. aureus, MRGR3, de void of a significant (>10(-7)) glycopeptide-resistant subpopulation at 2 m g/L of either teicoplanin or vancomycin. After 3 weeks of infection in anti biotic-untreated animals, subpopulations emerged, growing on agar containin g In mg/L of either glycopeptide. These subpopulations were detected in all tissue cage fluids containing >7 log cfu/ml at average frequencies of 4 x 10(-5) and 2 x 10-5 on teicoplanin- and vancomycin-containing agar, respect ively. While teicoplanin MICs increased two- to 16-fold, vancomycin MICs in creased by less than two-fold. population analysis and survival kinetic stu dies of three teicoplanin-selected subclones indicated that transfer from s olid to liquid medium conditions decreased expression of teicoplanin resist ance in the bacterial population. In Mueller-Hinton broth, >90% of cells re mained fully resistant to antibiotic, but did not grow in the presence of t eicoplanin for an initial period of at least 6 h. All three teicoplanin-res istant subclones expressed stable teicoplanin resistance with slight cross- resistance to vancomycin after a few transfers on teicoplanin-supplemented agar. These data suggest that some in vivo conditions may lead to selection of S. aureus subpopulations exhibiting decreased glycopeptide susceptibili ty and growing in the presence of otherwise inhibitory concentrations of th ese antimicrobial agents.