Frequency of biocide resistance genes, antibiotic resistance and the effect of chlorhexidine exposure on clinical methicillin-resistant Staphylococcus aureus isolates

Objectives: To detect genes conferring resistance to biguanides, quaternary ammonium compounds, beta-lactams and fluoroquinolones in clinical methicillin-resistant Staphylococcus aureus (MRSA) and to demonstrate whether reduced susceptibility is spread clonally and if the presence of any of the detected genes links to a specific epidemic MRSA. Finally, to identify if exposure to chlorhexidine may cause reduced susceptibility to antibiotics and chlorhexidine. Methods: In total, 120 clinical MRSA isolates were isolated. qacA/B, qacG, qacH, norA, smr and blaZ genes were amplified by PCR. MICs of eight antibiotics were determined and PFGE was used for typing. Surface disinfection and residue tests were performed for chlorhexidine and a selection of isolates. Results: qacA/B (8.3%), qacH (3.3%), norA (36.7%), smr (44.2%) and blaZ (97.5%) were prevalent within the population but qacG was not detected. EMRSA-15 (19.2%), EMRSA-16 (15%), P3 (15%) and H (12.5%) were the most common PFGE types. Clinical isolates demonstrated various degrees of susceptibility to chlorhexidine in the surface disinfection [mean microbiocidal effect (ME) = 0-1.91] and biocide residue (mean ME = 0.29-3.74) tests. Increases in post-exposure MICs were observed in both EMRSA-16 and the susceptible S. aureus control. Conclusions: In our study, isolates resembling PFGE type EMRSA-16 harboured more biocide resistance genes than other types. The observed reduction in susceptibility of clinical isolates to chlorhexidine may mean that a selective pressure is being exerted by residues in the clinical environment, and highlights the importance of efficacy testing on clinical strains and good infection control practices. The development of reduced microbial susceptibility to biocides represents a serious cause for concern in the clinical environment.