Vancomycin tolerance in methicillin-resistant Staphylococcus aureus: influence of vancomycin, daptomycin, and telavancin on differential resistance gene expression

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) isolates that are susceptible to vancomycin but are tolerant to its killing effect may present a potential challenge for effective treatment. This study compared the microbiologic characteristics of clinical vancomycin-tolerant (VT-MRSA) and vancomycin-susceptible (VS-MRSA) strains using phenotypic and gene regulation studies. MRSA isolates collected from vancomycin-treated patients with bacteremia over a 5-year period were analyzed for vancomycin, daptomycin, and telavancin susceptibility, as well as accessory gene regulator (agr) group and function. Vancomycin tolerance was defined by a minimum bactericidal concentration (MBC)/minimum inhibitor concentration (MIC) ratio of ≥32 mg/liter. VT-MRSA isolates were compared to VS-MRSA isolates for differences in antimicrobial susceptibility, time-kill activity, and gene expression of key cell envelope response genes vraSR, dltA, and mprF. All 115 isolates evaluated were susceptible to vancomycin, daptomycin, and telavancin. Seven isolates (6%) were VT-MRSA. agr group II was more prevalent in isolates with vancomycin MBC/MIC ratios of ≥8. In time-kill analyses, VT-MRSA had reduced vancomycin killing, but daptomycin and telavancin activities were maintained. Significantly greater gene expression was observed in VT-MRSA after 72 h of subinhibitory antibiotic exposures. Vancomycin most notably increased vraSR expression (P = 0.002 versus VS-MRSA strains). Daptomycin and telavancin increased expression of all genes studied, most significantly mprF expression (P < 0.001). Longer durations of antibiotic exposure (72 h versus 24 h) resulted in substantial increases in gene expression in VT-MRSA. Although the clinical impact of VT-MRSA is not fully recognized, these data suggest that VT-MRSA strains, while still susceptible, have altered gene regulation to adapt to the antimicrobial effects of glyco- and lipopeptides that may emerge during prolonged durations of exposure.

Fig 1

Comparison of accessory gene regulator (agr) group II and agr dysfunction in strains with high versus low vancomycin MBC/MIC ratios from patients with MRSA bacteremia.

Fig 2

Fold change in gene expression (mean ± standard deviation) of five vancomycin-tolerant MRSA strains (black bars) compared to five vancomycin-nontolerant MRSA strains (gray bars) following 72-h exposures to vancomycin (A), daptomycin (B), and telavancin (C).*, P < 0.05 between tolerant and susceptible strains; **, P < 0.001.

Fig 3

Fold change in gene expression in five vancomycin-tolerant strains after 24 h and 72 h of exposure to vancomycin (A), daptomycin (B), and telavancin (C). Upregulation of vraSR, dltA, and mprF correlated with the duration of antimicrobial exposure. *, P < 0.05 between gene expression at 72 h versus 24 h; **, P < 0.001.