Triclosan and antibiotic resistance in Staphylococcus aureus

Triclosan (2,4,4'-trichloro-2'-hydroxydiphenyl ether) is an antimicrobial agent used in hygiene products, plastics and kitchenware, and for treating methicillin-resistant Staphylococcus aureus (MRSA) outbreaks. S. aureus strains with low-level resistance to triclosan have emerged. It has been claimed that strains with decreased susceptibility to biocides may also be less susceptible to antibiotics. We tested the susceptibility of S. aureus clinical isolates to triclosan and several antibiotics. Triclosan MICs ranged between 0.025 and 1 mg/L. Some, but not all, strains were resistant to several antibiotics and showed low-level triclosan resistance. S. aureus mutants with enhanced resistance to triclosan (< or =1 mg/L) were isolated. In several cases this resistance was stably inherited in the absence of triclosan. These mutants were not more resistant than the parent strain to several antibiotics. Changes in triclosan MICs associated with the acquisition of a plasmid encoding mupirocin resistance were not observed, suggesting that the triclosan/mupirocin co-resistance seen in a previous study was not the result of a single resistance gene or separate genes on the same plasmid. The continuous exposure of a triclosan-sensitive S. aureus strain to sub-MIC concentrations of triclosan for 1 month did not result in decreased susceptibility to triclosan or to several antibiotics tested. Triclosan-induced potassium leakage and bactericidal effects on a triclosan-sensitive strain, a resistant strain and a strain selected for increased resistance were compared with those of non-growing organisms, exponentially growing organisms and organisms in the stationary phase. No significant differences between the strains were observed under these conditions despite their different MICs. Biocides have multiple target sites and so MICs often do not correlate with bactericidal activities. The ability of S. aureus to develop resistance to triclosan and the current view that triclosan may have a specific target in Escherichia coli, namely enoyl reductase, underline the need for more research on the mechanisms of action and resistance.