Understanding antimicrobial and antiseptic resistance relationships in Staphylococcus aureus

Abstract

Staphylococcus aureus is an opportunistic pathogen that causes skin infections, bacteraemia, and endocarditis in humans. In New Zealand, the common lineage of S. aureus is the ST1 lineage which harbours a fusidic acid resistance gene (fusC) on its chromosome and can carry the multi-drug resistance plasmid pNZAK1. pNZAK1 confers resistance to mupirocin (mupA), ampicillin (blaZ), and chlorhexidine (qacA). Chlorhexidine is a rapid and effective antiseptic widely used in healthcare to disinfect equipment and patients to prevent bacterial growth. The pNZAK1 plasmid encodes qacA, which produces a multi-drug efflux pump that can confer tolerance to chlorhexidine. The aim of this study was to investigate the maintenance of pNZAK1 in S. aureus 14487 using a continuous culture system to impose energy limiting conditions and examine the variation in antimicrobial sensitivity and viability. The second objective was to investigate the role of qacA using a ∆qacA mutant in antimicrobial sensitivity assays against an isogenic qacA+ wild-type, specifically looking at tolerance to biocides like chlorhexidine or benzalkonium chloride. Our results indicated that low levels of chlorhexidine can increase gene expression of qacA, mupA, and norA. Additionally, qacA can increase survival of bacteria under antimicrobial pressure from biocide mixtures like Trigene. These data also showed that pNZAK1 was highly stable (>94%) in S. aureus 14487 and was maintained even without selection pressure under generalised nutrient limitation. Bacteria that lost pNZAK1 (5-6%) were isolated and showed an increased antimicrobial sensitivity. To investigate key mechanisms of plasmid maintenance we focused on the bacteriocin lactococcin 972, and the toxin/antitoxin system pepA1. By creating single gene knockouts we observed no change in pNZAK1 plasmid maintenance. These results suggest that pNZAK1 is a highly stable multi-drug resistant plasmid and that plasmid-encoded genes such as qacA can be induced by sub-MIC levels of commonly used antiseptics such as chlorhexidine and can be beneficial to survival.