title: Low selection of topoisomerase mutants from strains of Escherichia coli harbouring plasmid-borne qnr genes.
abstract: OBJECTIVE
  To investigate mutations in the type II topoisomerase genes in quinolone-resistant mutants selected from bacteria harbouring plasmid-borne qnr genes.
  
  
  METHODS
  Mutants were selected by nalidixic acid, ciprofloxacin and moxifloxacin from two Escherichia coli reference strains and corresponding transconjugants harbouring qnrA1, qnrA3, qnrB2 or qnrS1 genes.
  
  
  RESULTS
  The proportion of resistant mutants selected by the three quinolones was, respectively, in the same range for qnr-positive transconjugants and reference strains. Only 20% (65/329) of the mutants selected from the transconjugants showed a gyrase mutation, whereas 79% (94/119) of those from the reference strains without a qnr gene did (P < 0.0001). At four times the MIC of the selector quinolone, gyrA mutants represented 49% and 95% of the mutants selected with nalidixic acid, 4% and 94% with ciprofloxacin and 0% and 54% with moxifloxacin for qnr-positive transconjugants and reference strains, respectively. Mutations within gyrA were distributed at codon 87 (D87G, H, N or Y) and at codon 83 (S83L) with three novel mutations (gyrA Ser83stop, gyrA Asp82Asn and gyrB insertion of Glu at 465) and three rare mutations (gyrA Gly81Asp, gyrA Asp82Gly and gyrA Ser431Pro), mainly obtained from reference strains after moxifloxacin selection. Strikingly, none of the mutants selected by moxifloxacin from qnr-positive transconjugants harboured a mutation in the topoisomerase genes.
  
  
  CONCLUSIONS
  Topoisomerase mutants are rarely selected by ciprofloxacin and moxifloxacin from strains harbouring qnr. This suggests that the quinolone resistance-determining region domains are protected from quinolones by the Qnr protein and consequently other mechanisms are developed to acquire a further step of fluoroquinolone resistance.
authors.affiliation: Université Paris12, IFR10, Créteil, France.
authors.forename: Annabelle
authors.initials: A
authors.lastname: Cesaro
authors.forename: Romain Roth Dit
authors.initials: RR
authors.lastname: Bettoni
authors.forename: Christine
authors.initials: C
authors.lastname: Lascols
authors.forename: Audrey
authors.initials: A
authors.lastname: Mérens
authors.forename: Claude James
authors.initials: CJ
authors.lastname: Soussy
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Cambau
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D003062:Codon / Q000235:genetics; D004250:DNA Topoisomerases, Type II / Q000235:genetics*; D024901:Drug Resistance, Multiple, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000145:classification / Q000201:enzymology* / Q000235:genetics*; D029968:Escherichia coli Proteins / Q000235:genetics*; D015964:Gene Expression Regulation, Bacterial; D008826:Microbial Sensitivity Tests; D009154:Mutation; D010957:Plasmids / Q000235:genetics*; D015363:Quinolones / Q000494:pharmacology; D012641:Selection, Genetic
chemical_list: D000900:Anti-Bacterial Agents; D003062:Codon; D029968:Escherichia coli Proteins; C485384:Qnr protein, E coli; D015363:Quinolones; D004250:DNA Topoisomerases, Type II