Sustained chlorhexidine susceptibility in clinical MRSA isolates despite recent exposure.

Background and objectives: MRSA decolonization strategies are widely implemented to reduce the risk of invasive infections but rely heavily on the use of chlorhexidine. Reduced chlorhexidine susceptibility has been reported after widespread use in hospital settings. The objective of the study is to assess chlorhexidine susceptibility among clinical MRSA isolates and to investigate potential induction of reduced susceptibility following chlorhexidine exposure. Methods: Ninety-five MRSA isolates from 77 patients of the University Hospital Zurich, treated between October 2021 and September 2022, were included. Chlorhexidine MICs were determined using the broth microdilution method, and subsequently MBCs were assessed. WGS was performed using the Illumina technology. Ten isolates with MBC values ≥8 mg/L were selected for cyclic exposure experiments using sub-inhibitory chlorhexidine concentrations and for analysis of genes associated with reduced chlorhexidine susceptibility. Longitudinal isolates from seven patients were analysed to assess in-host evolution. Results: Most isolates showed chlorhexidine MICs of 2 mg/L (76.6%, 60/77) and MBCs of 4 mg/L (68.8%, 53/77). ST8 was the most represented MLST among the isolates tested (23.1%). None of the 10 isolates with MBCs ≥8 mg/L demonstrated further reductions in chlorhexidine susceptibility following repeated sub-lethal exposure. Although resistance-associated genes were detected, no phenotypic impact on chlorhexidine susceptibility was observed. Longitudinal analysis of seven patients revealed stable MIC and MBC values despite prolonged carriage and chlorhexidine exposure. Conclusions: These findings indicate the absence of inducible or clinically relevant reduced susceptibility to chlorhexidine among MRSA isolates in this setting, despite repeated exposure.