title: Ciprofloxacin selects for multidrug resistance in Salmonella enterica serovar Typhimurium mediated by at least two different pathways.
abstract: OBJECTIVE
  The aim of this study was to understand the role of ramA in conferring multidrug resistance (MDR) in Salmonella enterica serovar Typhimurium.
  
  
  METHODS
  Two phenotypically distinct isogenic MDR laboratory mutants derived from Salmonella Typhimurium SL1344 and three human clinical isolates from a patient that failed antibiotic therapy, including with ciprofloxacin, were investigated for the cause of MDR. MICs were determined by agar dilution and efflux activity assessed by monitoring the accumulation of Hoechst 33342. The combination of specific genes and MDR was assessed by inactivation, and complementation RT-PCR was used to assess gene expression and DNA sequencing to identify mutations within genes of interest.
  
  
  RESULTS
  Mutation in ramR and the consequent over-production of ramA and acrB were revealed in one laboratory mutant selected with ciprofloxacin and this was associated with cyclohexane tolerance. Complementation of SL1344 ramR::aph with pUC19 ramR(mutant) conferred MDR and cyclohexane tolerance. However, analysis of a second ciprofloxacin-selected MDR mutant, which was susceptible to cyclohexane, revealed no mutation in ramR or altered expression of marA, soxS or rob. There was a mutation in ramR in both the pre- and post-therapy clinical isolates and no difference between the isolates in the level of expression of ramA.
  
  
  CONCLUSIONS
  These data show that ciprofloxacin exposure can select for mutations within ramR and consequently mediate MDR. These data also indicate that there is at least one further unidentified gene in addition to marA, soxS, rob and ramA that confers MDR in S. enterica.
authors.affiliation: University of Birmingham, UK.
authors.forename: Vito
authors.initials: V
authors.lastname: Ricci
authors.forename: Laura J V
authors.initials: LJ
authors.lastname: Piddock
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001562:Benzimidazoles / Q000378:metabolism; D002939:Ciprofloxacin / Q000494:pharmacology*; D024901:Drug Resistance, Multiple, Bacterial; D017353:Gene Deletion; D020869:Gene Expression Profiling; D005816:Genetic Complementation Test; D006801:Humans; D008826:Microbial Sensitivity Tests; D027425:ATP-Binding Cassette, Sub-Family C Proteins / Q000235:genetics / Q000378:metabolism; D012486:Salmonella typhimurium / Q000187:drug effects* / Q000235:genetics*; D012641:Selection, Genetic; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D001562:Benzimidazoles; D002939:Ciprofloxacin; D027425:ATP-Binding Cassette, Sub-Family C Proteins; D015534:Trans-Activators; C531999:RamA protein, Salmonella typhimurium; C485381:SoxS protein, Salmonella enterica; C017807:bisbenzimide ethoxide trihydrochloride