title: Ciprofloxacin-resistant Salmonella enterica serovar Typhimurium strains are difficult to select in the absence of AcrB and TolC.
abstract: It has been proposed that lack of a functional efflux system(s) will lead to a lower frequency of selection of resistance to fluoroquinolones and other antibiotics. We constructed five strains of Salmonella enterica serovar Typhimurium SL1344 that lacked efflux gene components of resistance nodulation cell division pumps (acrB, acrD, acrF, acrBacrF, and tolC) plus three strains that lack genes that effect efflux gene expression (marA, soxS, and ramA) and a hypermutable strain (mutS::aph). Strains were exposed to ciprofloxacin at 2x the MIC in agar, in the presence and absence of Phe-Arg-beta-naphthylamide, an efflux pump inhibitor. Mutants were selected from all strains except those lacking acrB, tolC, or acrBacrF. For strains from which mutants were selected, there were no significant differences between the frequencies of resistance. Except for mutants of the ramA::aph strain, two phenotypes arose: resistance to quinolones only and multiple antibiotic resistance (MAR). ramA::aph mutants were resistant to quinolones only, suggesting a role for ramA in MAR in S. enterica serovar Typhimurium. Phe-Arg-beta-naphthylamide (20 microg/ml) had no effect on the frequencies of resistance or ciprofloxacin MICs. In conclusion, functional AcrB and TolC in S. enterica serovar Typhimurium are important for the selection of ciprofloxacin-resistant mutants.
authors.affiliation: Antimicrobial Agents Research Group, Division of Immunity and Infection, University of Birmingham, Birmingham B15 2TT, United Kingdom.
authors.forename: Vito
authors.initials: V
authors.lastname: Ricci
authors.forename: Peter
authors.initials: P
authors.lastname: Tzakas
authors.forename: Anthony
authors.initials: A
authors.lastname: Buckley
authors.forename: Laura J V
authors.initials: LJ
authors.lastname: Piddock
mesh_terms: D000890:Anti-Infective Agents / Q000494:pharmacology*; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002939:Ciprofloxacin / Q000378:metabolism / Q000494:pharmacology*; D024901:Drug Resistance, Multiple, Bacterial / Q000235:genetics*; D029968:Escherichia coli Proteins / Q000378:metabolism*; D005798:Genes, Bacterial / Q000235:genetics; D026901:Membrane Transport Proteins; D019779:Salmonella enterica / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism / Q000502:physiology*
chemical_list: C474926:AcrB protein, Salmonella enterica; D000890:Anti-Infective Agents; D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; C073126:tolC protein, E coli; D002939:Ciprofloxacin