The Binding of Triclosan to SmeT, the Repressor of the Multidrug Efflux Pump SmeDEF, Induces Antibiotic Resistance in Stenotrophomonas maltophilia

Abstract

The wide utilization of biocides poses a concern on the impact of these compounds on natural bacterial populations. Furthermore, it has been demonstrated that biocides can select, at least in laboratory experiments, antibiotic resistant bacteria. This situation has raised concerns, not just on scientists and clinicians, but also on regulatory agencies, which are demanding studies on the impact that the utilization of biocides may have on the development on resistance and consequently on the treatment of infectious diseases and on human health. In the present article, we explored the possibility that the widely used biocide triclosan might induce antibiotic resistance using as a model the opportunistic pathogen Stenotrophomonas maltophilia. Biochemical, functional and structural studies were performed, focusing on SmeDEF, the most relevant antibiotic- and triclosan-removing multidrug efflux pump of S. maltophilia. Expression of smeDEF is regulated by the repressor SmeT. Triclosan released SmeT from its operator and induces the expression of smeDEF, thus reducing the susceptibility of S. maltophilia to antibiotics in the presence of the biocide. The structure of SmeT bound to triclosan is described. Two molecules of triclosan were found to bind to one subunit of the SmeT homodimer. The binding of the biocide stabilizes the N terminal domain of both subunits in a conformation unable to bind DNA. To our knowledge this is the first crystal structure obtained for a transcriptional regulator bound to triclosan. This work provides the molecular basis for understanding the mechanisms allowing the induction of phenotypic resistance to antibiotics by triclosan. The wide utilization of biocides for different purposes, including toothpastes, soaps, house-hold compounds surfaces' disinfectants and even their use as additives of different materials (from textiles to concrete used in germ-free buildings) to avoid their colonization by microorganisms, poses a concern on the impact of these compounds on natural bacterial populations. Furthermore, it has been demonstrated that such biocides can select, at least in laboratory experiments, bacteria resistant to antibiotics. This situation has raised concerns on the impact that the utilization of biocides may have on the development on resistance and consequently on the treatment of infectious diseases. In the present article we study whether biocides can induce phenotypic resistance to antibiotics, a process that would be barely detectable unless purposely searched out. In the article, we present functional, biochemical and structural data showing that the widely used biocide triclosan induces antibiotic resistance, mediated by the binding of the biocide to SmeT, the transcriptional regulator of the expression of the Stenotrophomonas maltophilia multidrug efflux pump SmeDEF, which can extrude an ample range of antibiotics. Our study provides an unambiguous link between the presence of this biocide and the increased efflux of antibiotics by the opportunistic pathogen S. maltophilia.