Staphylococcus aureus but not Listeria monocytogenes adapt to triclosan and adaptation correlates with increased fabI expression and agr deficiency

Lene Nørby Nielsen,Lone Gram,Sissel Skovgaard,Vicky Kastbjerg,Henrik Westh,Hanne Ingmer,Marianne Halberg Larsen

doi:10.1186/1471-2180-13-177

Abstract

BackgroundThe ability of pathogens to adapt to the widely used biocide, triclosan, varies substantially. The purpose of the study was to examine bacterial adaptation over an extended period of time to low increments of triclosan concentrations. Focus was two human pathogens, S. aureus and L. monocytogenes that previously have displayed inherent high and low adaptability, respectively.ResultsThree strains of L. monocytogenes and two strains of S. aureus including the community-acquired USA300 were exposed to increasing, sub-lethal concentrations of triclosan in triclosan-containing agar gradients. Following 25 days of exposure on agar plates to sub-lethal concentrations of triclosan with a twofold concentration increase every second day, minimum inhibitory concentration (MIC) for S. aureus increased from 0.125 (8325–4) and 0.0625 (USA 300) mg/L to 4 mg/L. The MIC of all three L. monocytogenes strains was initially 4 mg/L and remained unaltered by the exposure. The adapted S. aureus isolates retained normal colony size but displayed increased expression of fabI encoding an essential enzyme in bacterial fatty acid synthesis. Also, they displayed decreased or no expression of the virulence associated agrC of the agr quorum sensing system. While most adapted strains of USA300 carried mutations in fabI, none of the adapted strains of 8325–4 did.ConclusionsAdaptability to triclosan varies substantially between Gram positive human pathogens. S. aureus displayed an intrinsically lower MIC for triclosan compared to L. monocytogenes but was easily adapted leading to the same MIC as L. monocytogenes. Even though all adapted S. aureus strains over-expressed fabI and eliminated expression of the agr quorum sensing system, adaptation in USA300 involved fabI mutations whereas this was not the case for 8325–4. Thus, adaptation to triclosan by S. aureus appears to involve multiple genetic pathways.

Highlights

The ability of pathogens to adapt to the widely used biocide, triclosan, varies substantially
In these food associated bacteria including pathogens such as Listeria monocytogenes may be repeatedly exposed to the compound [4]
S. aureus strains were grown in Mueller Hinton (MH) broth (Oxoid, Denmark) or Tryptic Soy broth (TSB)

Summary

Introduction

The ability of pathogens to adapt to the widely used biocide, triclosan, varies substantially. Full list of author information is available at the end of the article reduce bacterial contamination in many cosmetics [1,2] These products typically contain concentrations of triclosan ranging from 0.1 to 0.3% by weight, corresponding to 1000 to 3000 mg/L [1]. Triclosan has been incorporated into different types of plastics including food storage containers and chopping boards [3]. In these food associated bacteria including pathogens such as Listeria monocytogenes may be repeatedly exposed to the compound [4]. In the hospital environment pathogens may encounter triclosan when it is employed in surgical

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