Many health care centers have reported an association between Staphylococcus aureus isolates bearing efflux pump genes and an elevated MIC/minimal bactericidal concentration (MBC) to chlorhexidine gluconate (CHG) and other antiseptics. The significance of these organisms is uncertain, given that their MIC/MBC is typically far lower than the CHG concentration in most commercial preparations. We sought to evaluate the relationship between carriage of the efflux pump genes qacA/B and smr in S. aureus and the efficacy of CHG-based antisepsis in a venous catheter disinfection model. S. aureus isolates with and without smr and/or qacA/B were utilized. The CHG MICs were determined. Venous catheter hubs were inoculated and exposed to CHG, isopropanol, and CHG-isopropanol combinations. The microbiocidal effect was calculated as the percent reduction in CFU following exposure to the antiseptic relative to the control. The qacA/B- and smr-positive isolates had modest elevations in the CHG MIC90 compared to the qacA/B- and smr-negative isolates (0.125 mcg/ml vs. 0.06 mcg/ml, respectively). However, the CHG microbiocidal effect was significantly lower for qacA/B- and/or smr-positive strains than for susceptible isolates, even when the isolates were exposed to CHG concentrations up to 400 μg/mL (0.04%); this finding was most notable for isolates bearing both qacA/B and smr (89.3% versus 99.9% for the qacA/B- and smr-negative isolates; P = 0.04). Reductions in the median microbiocidal effect were also observed when these qacA/B- and smr-positive isolates were exposed to a solution of 400 μg/mL (0.04%) CHG and 70% isopropanol (89.5% versus 100% for the qacA/B- and smr-negative isolates; P = 0.002). qacA/B- and smr-positive S. aureus isolates have a survival advantage in the presence of CHG concentrations exceeding the MIC. These data suggest that traditional MIC/MBC testing may underestimate the ability of these organisms to resist the effects of CHG. IMPORTANCE Antiseptic agents, including chlorhexidine gluconate (CHG), are commonly utilized in the health care environment to reduce rates of health care-associated infections. A number of efflux pump genes, including smr and qacA/B, have been reported in Staphylococcus aureus isolates that are associated with higher MICs and minimum bactericidal concentrations (MBCs) to CHG. Several health care centers have reported an increase in the prevalence of these S. aureus strains following an escalation of CHG use in the hospital environment. The clinical significance of these organisms, however, is uncertain, given that the CHG MIC/MBC is far below the concentration in commercial preparations. We present the results of a novel surface disinfection assay utilizing venous catheter hubs. We found that qacA/B-positive and smr-positive S. aureus isolates resist killing by CHG at concentrations far exceeding the MIC/MBC in our model. These findings highlight that traditional MIC/MBC testing is insufficient to evaluate susceptibility to antimicrobials acting on medical devices.
Keywords: Staphylococcus aureus; chlorhexidine gluconate; disinfection; pediatric; qacA/B; smr.