The biocide triclosan triggers multiple regulatory systems in Staphylococcus aureus to induce antibiotic tolerance

The biocide triclosan is used extensively in household and hospital settings, resulting in chronic exposure to the biocide in individuals that use triclosan-containing products. Triclosan is thought to induce antibiotic tolerance and alter biofilm formation, although the underlying mechanisms causing these changes are yet to be elucidated. If true, the widely used biocide may contribute to antibiotic treatment failures, and therefore requires investigation. To determine how triclosan induces antibiotic tolerance, Staphylococcus aureus was pre-treated with triclosan prior to treatment with the clinically relevant antibiotics ciprofloxacin, rifampicin, and vancomycin. Planktonic S. aureus cultures pre-treated with triclosan had 1000 fold higher viable counts compared to non triclosan pre-treated cultures. Inspection of biofilms by live/dead staining found that triclosan pre-treatment protected S. aureus biofilms from treatment with otherwise lethal doses of ciprofloxacin, rifampicin, or vancomycin. Biofilms of mutant strains with a defective stringent response were not protected from antibiotic treatment, even in the presence of triclosan. Interestingly, stringent response mutants still exhibited triclosan-induced antibiotic tolerance in planktonic culture, but mutants with a defective agr quorum sensing system did not. Confocal laser scanning microscopy revealed that incubation of S. aureus with triclosan altered biofilm structure, resulting in increased proportions of polysaccharide in the biofilm matrix that could potentially mediate protection against antibiotics. Neither the stringent response mutants nor agr mutants influenced triclosan-induced biofilm changes, suggesting another, currently uncharacterised response. We suggest that triclosan triggers multiple global regulatory systems in S. aureus, subsequently inducing tolerance to multiple antibiotic classes and altering biofilm structure.