Abstract

Hospital acquired infection is a major cause of morbidity and mortality and regimes to prevent infection are crucial in infection control. These include decolonisation of at-risk patients of carriage of MRSA which is commonly achieved by protocols that include the use of chlorhexidine, or octenidine as biocidal agents. There is however no standardised single decolonisation regime agreed upon in the UK or other countries and protocols include a variety of active agents. Antibiotic resistant bacteria cause major problems in hospital medicine and concern has been raised regarding the development of biocide resistance which would cause decolonisation regimes to become unreliable. In this study, we assembled a panel of isolates of S. aureus including isolates collected before the development of chlorhexidine and octenidine through to a contemporaneous panel of isolates from a major hospital trust in the UK during a period when the decolonisation regime was altered. We observed significant increases in the MIC and MBC of chlorhexidine in isolates collected from periods of high usage of chlorhexidine. No isolates had a significantly altered MIC or MBC of octenidine apart from those collected after octenidine was introduced into the trust where isolates with four-fold decreases in susceptibility emerged. There was no suggestion of cross-resistance between the two biocidal agents. A combination of VNTR, PCR for qac genes and whole genome sequencing was used to type isolates and examine possible mechanisms of resistance. The typing data showed no expansion of a single strain was associated with decreased biocide tolerance and isolates with increased chlorhexidine MIC and MBCs were found from different clonal complexes; CC8, CC22 and CC30. Biocide susceptibility did not correlate with carriage of qac efflux pump genes - carriage of qacA and qacB was detected but, with one exception was restricted to isolates of CC8. Analysis of genome sequence data for closely related pairs of strains with differential biocide susceptibility revealed no common mutations or carriage of accessory elements that correlated with biocide tolerance. Mutations with the NorA or NorB efflux pumps, previously associated with chlorhexidine export were identified suggesting this may be an important mechanism of biocide tolerance. The clinical relevance of decreased biocide tolerance in terms of efficacy of decolonisation therapies remains to be established but we present evidence here that isolates are evolving in the face of biocide challenge in patients and that changes to decolonisation regimes are reflected in changes in susceptibility of isolates. More work is needed to assess the impact of these changes to ensure effective and robust decolonisation protocols remain in place.