title: Importance of sigma factor mutations in increased triclosan resistance in Salmonella Typhimurium.
abstract: BACKGROUND
  Salmonella enterica is the second most common foodborne pathogen. The use of biocides is crucial to prevent spread of foodborne pathogens, and it would be devastating for food safety if Salmonella would become resistant to the disinfectants used. Another concern is that exposure to disinfectants might lead to decreased susceptibility to antibiotics. The current study aimed to identify genetic changes causing high level triclosan resistance in S. enterica serovar Typhimurium and evaluate how these affected antibiotic resistance and efflux pump activity.
  
  
  RESULTS
  Wild type strains S. Typhimurium 4/74 and DTU3 were adapted to increasing concentrations of the biocide triclosan by serial passage. High level triclosan resistant isolates (MIC > 1000 μg/ml) were obtained. Strains were genome sequenced, and SNPs in fabI, rpoS and rpoD were found to be associated with high level resistance. However, work with defined mutants revealed that a SNP in fabI was not sufficient to obtain high level resistance. This required additional mutations in the sigma factors rpoS or rpoD. The adapted strains showed triclosan-dependent increased efflux, increased fabI expression and reduced susceptibility towards the antibiotics enrofloxacin and sulphamethoxazole/trimethoprim.
  
  
  CONCLUSIONS
  Medium level triclosan resistance could be obtained by fabI mutations in S. Typhimurium, however, high level resistance was found to require sigma factor mutations in addition to a fabI mutation. Reduced antibiotic sensitivity was observed for the adapted strains, which could be associated with increased efflux.
authors.affiliation: Department of Veterinary Disease Biology, University of Copenhagen, Faculty of Health and Medical Sciences, Stigboejlen 4, 1870, Frederiksberg C, Denmark. mettergantzhorn@gmail.com.
authors.forename: Mette Rørbæk
authors.initials: MR
authors.lastname: Gantzhorn
authors.affiliation: Department of Veterinary Disease Biology, University of Copenhagen, Faculty of Health and Medical Sciences, Stigboejlen 4, 1870, Frederiksberg C, Denmark. jeo@sund.ku.dk.
authors.forename: John Elmerdahl
authors.initials: JE
authors.lastname: Olsen
authors.affiliation: Department of Veterinary Disease Biology, University of Copenhagen, Faculty of Health and Medical Sciences, Stigboejlen 4, 1870, Frederiksberg C, Denmark. leth@sund.ku.dk.
authors.forename: Line Elnif
authors.initials: LE
authors.lastname: Thomsen
mesh_terms: D004252:DNA Mutational Analysis; D004202:Disinfectants / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D050753:Enoyl-(Acyl-Carrier-Protein) Reductase (NADH) / Q000235:genetics; D016680:Genome, Bacterial; D008826:Microbial Sensitivity Tests; D020125:Mutation, Missense; D012486:Salmonella typhimurium / Q000187:drug effects* / Q000235:genetics* / Q000254:growth & development; D017422:Sequence Analysis, DNA; D012692:Serial Passage; D012808:Sigma Factor / Q000235:genetics*; D014260:Triclosan / Q000494:pharmacology*
chemical_list: D004202:Disinfectants; D012808:Sigma Factor; D014260:Triclosan; D050753:Enoyl-(Acyl-Carrier-Protein) Reductase (NADH)