A novel qacA allele results in an elevated chlorhexidine gluconate minimum inhibitory concentration in cutaneous Staphylococcus epidermidis isolates

Abstract

Chlorhexidine gluconate (CHG) is a topical antiseptic widely used in healthcare settings. In Staphylococcus spp., the pump QacA effluxes CHG, while the closely related QacB cannot due to a single amino acid substitution. We characterized 1,050 cutaneous Staphylococcus isolates obtained from 173 pediatric oncology patients enrolled in a multicenter CHG bathing trial. CHG susceptibility testing revealed 63 (6%) of these isolates had elevated CHG MICs ([≥] 4 g/mL). Screening of all 1,050 isolates for qacA/B by restriction fragment length polymorphism (RFLP) yielded 56 isolates with a novel qacA/B RFLP pattern, qacAB273. The CHG MIC was significantly higher for qacAB273-positive isolates (MIC50: 4 g/mL, [range: 0.5 - 4 g/mL]) compared to other qac groups: qacA-positive (n=559, 1 g/mL, [0.5 - 4 g/mL]), qacB-positive (n=17, 1 g/mL, [0.25 - 2 g/mL]), and qacA/B-negative (n=418, 1 g/mL, [0.125 - 2 g/mL], p=0.001). The qacAB273-positive isolates also displayed a high proportion of methicillin resistance (96.4%) compared to other qac groups (24.9 - 61.7%, p=0.001). Whole genome sequencing revealed that qacAB273-positive isolates encoded a variant of QacA with 2 amino acid substitutions. This new allele, named qacA4, was carried on the novel plasmid pAQZ1. The qacA4-carrying isolates belonged to the highly resistant S. epidermidis clone ST2 and were collected from multiple centers across the United States and Canada. Curing an isolate of qacA4 resulted in a four-fold decrease in the CHG MIC, confirming the role of qacA4 in the elevated CHG MIC. Our results highlight the importance of further studying qacA4 and its functional role in clinical staphylococci.\n\nImportanceStaphylococcus epidermidis is an important cause of infections in patients with implanted devices. Bathing with chlorhexidine gluconate (CHG), a topical antiseptic, has been shown to reduce rates of device-associated infections, especially those caused by S. epidermidis. In S. epidermidis, reduced susceptibility to CHG is associated with carriage of the qacA gene. As part of a multicenter CHG bathing trial, we obtained cutaneous Staphylococcus isolates from pediatric oncology patients across the United States and Canada. We identified a group of isolates capable of surviving in higher concentrations of CHG and determined a novel allele of qacA, termed qacA4 and carried on the novel plasmid pAQZ1, was responsible for the isolates survival in higher CHG concentrations. The qacA4-carrying S. epidermidis isolates belonged to the highly resistant and virulent ST2 clonal type. Our results highlight the need to understand the global distribution of novel qacA alleles, including qacA4, and their mechanistic effect on efflux.