Physicochemical factors affecting the rapid bactericidal efficacy of the phenolic antibacterial triclosan

The antimicrobial activity of triclosan (TCS; 2,4,4'-trichloro-2'-hydroxydiphenyl ether) in aqueous solutions is shown to directly depend upon two key physicochemical parameters: % saturation and saturation solubility. Saturated solutions of TCS in water, water-propylene glycol (PG) mixtures, and aqueous surfactant systems are shown to effect rapid, potent bacterial reductions (e.g. >4 log(10) reduction of Staphylococcus aureus in 15 s contact time in a time kill suspension test). In surfactant solutions, increasing the surfactant: TCS ratio causes a decrease in antibacterial efficacy, consistent with a model for micellar solubilization where the micelle binding constant, K (=X/c(w)) increases with decreasing TCS concentration in the micelles (X), resulting in decreased concentration of bioavailable TCS in the water (continuous) phase (c(w)). The rapid and potent reductions of bacteria reported here were surprising and support the existence of a non-specific mode of action for TCS, such as gross membrane disruption, in addition to the specific modes of action reported by others.