Defining and Combating the Mechanisms of Triclosan Resistance in Clinical Isolates ofStaphylococcus aureus

Abstract

The MICs of triclosan for 31 clinical isolates of Staphylococcus aureus were 0.016 g/ml (24 strains), 1 to 2 g/ml (6 strains), and 0.25 g/ml (1 strain). All the strains for which triclosan MICs were elevated (>0.016 g/ml) showed three-to fivefold increases in their levels of enoyl-acyl carrier protein (ACP) reductase (FabI) production. Furthermore, strains for which triclosan MICs were 1 to 2 g/ml overexpressed FabI with an F204C alteration. Binding studies with radiolabeled NAD ؉ demonstrated that this change prevents the formation of the stable triclosan-NAD ؉ -FabI complex, and both this alteration and its overexpression contributed to achieving MICs of 1 to 2 g/ml for these strains. Three novel, potent inhibitors of FabI (50% inhibitory concentrations, <64 nM) demonstrated up to 1,000-fold better activity than triclosan against the strains for which triclosan MICs were elevated. None of the compounds tested from this series formed a stable complex with NAD ؉ -FabI. Consequently, although the overexpression of wild-type FabI gave rise to an increase in the MICs, as expected, overexpression of FabI with an F204C alteration did not cause an additional increase in resistance. Therefore, this work identifies the mechanisms of triclosan resistance in S. aureus, and we present three compounds from a novel chemical series of FabI inhibitors which have excellent activities against both triclosan-resistant and -sensitive clinical isolates of S. aureus.Initially, it was thought that the mode of antibacterial action of triclosan was via a general disruptive effect on bacterial membranes (15). However, it has now been shown that the mode of action of triclosan is via inhibition of enoyl-acyl carrier protein (ACP) reductase (FabI) (11) in organisms such as Staphylococcus aureus and Escherichia coli that rely on this enzyme to perform the ultimate step in the elongation cycle of bacterial fatty acid biosynthesis (3,4,5,11,16). Resistance to triclosan has been the subject of much discussion in recent years, and laboratory studies with E. coli and S. aureus have shown that mutations in FabI (G93V/S and G23S, respectively) and their overexpression cause decreases in susceptibility to triclosan (3,4,5,11,16). However, characterization of the triclosan resistance in clinically derived isolates of either organism has not yet been reported.Triclosan exhibits exquisite activity against S. aureus and is used to control the carriage of methicillin-resistant S. aureus in hospitals (1). However, despite debate on the use of triclosan, few surveys have evaluated the level of resistance to triclosan in this important pathogen. Furthermore, the mechanism of triclosan resistance in clinically derived isolates of S. aureus has not been reported. Consequently, in this work we have identified a set of triclosan-resistant isolates of S. aureus and elucidated the mechanisms that give rise to reduced susceptibility to triclosan. Furthermore, we present some novel inhibitors of FabI which are active against these triclosan-resistant isolates of ...

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