Effect of outer membrane permeabilisation on intrinsic resistance to low triclosan levels in Pseudomonas aeruginosa

The present study was undertaken to investigate the possibility that outer cell envelope impermeability might be involved in the intrinsic resistance of Pseudomonas aeruginosa to low levels of the hydrophobic biocide triclosan. Macrobroth dilution and batch cultural turbidimetric assays were employed to assess the ability of compounds that render the Gram-negative outer membrane permeable to non-polar molecules to sensitise cell envelope variants to triclosan. Pseudomonas aeruginosa strains possessing highly refractory (PAO1) and atypically permeable (Z61) outer cell envelopes as well as a PAO1 derivative lacking four multidrug efflux pumps (YM64) were examined. Whilst the triclosan minimal inhibitory concentrations (MICs) differed dramatically for both PAO1 and Z61, significant decreases were seen for both strains in the presence of the outer membrane permeabiliser polymyxin B-nonapeptide. Strain YM64 was as resistant to triclosan as strain PAO1. Turbidimetric assessments of batch cultural growth kinetics revealed that the three chemically unrelated outer membrane permeabilisers polymyxin B-nonapeptide, compound 48/80 and ethylenediaminetetraacetate (EDTA) sensitised all strains to a sub-MIC concentration of triclosan (2.0 μg/mL). These data support the notion that the outer membrane exclusionary properties of P. aeruginosa for non-polar molecules confer intrinsic resistance to low concentrations of triclosan such as might be expected to occur in environmental residues. Moreover, a role for outer cell envelope impermeability is suggested for resistance to high triclosan concentrations in vitro.