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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.

Importance of sigma factor mutations in increased triclosan resistance in Salmonella Typhimurium