Open Access Highly Accessed Research article

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

Lene Nørby Nielsen1, Marianne Halberg Larsen1, Sissel Skovgaard12, Vicky Kastbjerg4, Henrik Westh23, Lone Gram4 and Hanne Ingmer1*

Author Affiliations

1 Department of Veterinary Disease Biology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark

2 Hvidovre Hospital, Department of Clinical Microbiology, Copenhagen, Denmark

3 Department of Clinical Medicin, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark

4 National Food Institute, Technical University of Denmark, Copenhagen, Denmark

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BMC Microbiology 2013, 13:177  doi:10.1186/1471-2180-13-177

Published: 30 July 2013



The 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.


Three 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.


Adaptability 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.

Staphylocooccus aureus; Listeria monocytogenes; Adaptation; Triclosan