title: Fluoroquinolone treatment of experimental Salmonella enterica serovar Typhimurium DT104 infections in chickens selects for both gyrA mutations and changes in efflux pump gene expression.
abstract: OBJECTIVE
  To determine the efficacy of enrofloxacin (Baytril) in chickens in eradicating three different resistance phenotypes of Salmonella enterica and to examine the resistance mechanisms of resulting mutants.
  
  
  METHODS
  In two separate replicate experiments (I and II), three strains of Salmonella enterica serovar Typhimurium DT104 [strain A, fully antibiotic-sensitive strain; strain B, isogenic multiple antibiotic-resistant (MAR) derivative of A; strain C, veterinary penta-resistant phenotype strain containing GyrA Phe-83], were inoculated into day-old chicks at approximately 10(3) cfu/bird. At day 10, groups of chicks (n =10) were given either enrofloxacin at 50 ppm in their drinking water for 5 days or water alone (control). Caecal contents were monitored for presence of Salmonella and colonies were replica plated to media containing antibiotics or overlaid with cyclohexane to determine the proportion of isolates with reduced susceptibility. The MICs of antibiotics and cyclohexane tolerance were determined for selected isolates from the chicks. Mutations in topoisomerase genes were examined by DHPLC and expression of marA, soxS, acrB, acrD and acrF by RT-PCR.
  
  
  RESULTS
  In experiment I, but not II, enrofloxacin significantly reduced the numbers of strain A compared with the untreated control group. In experiment II, but not I, enrofloxacin significantly reduced the numbers of strain B. Shedding of strain C was unaffected by enrofloxacin treatment. Birds infected with strains A and B gave rise to isolates with decreased fluoroquinolone susceptibility. Isolates derived from strain A or B requiring >128 mg/L nalidixic acid for inhibition contained GyrA Asn-82 or Phe-83. Isolates inhibited by 16 mg/L nalidixic acid were also less susceptible to antibiotics of other chemical classes and became cyclohexane-tolerant (e.g. MAR).
  
  
  CONCLUSIONS
  These studies demonstrate that recommended enrofloxacin treatment of chicks rapidly selects for strains with reduced fluoroquinolone susceptibility from fully sensitive and MAR strains. It can also select for MAR isolates.
authors.affiliation: Department of Food and Environmental Safety, Veterinary Laboratories Agency, New Haw, Surrey KT15 3NB, UK. l.randall@vla.defra.gsi.gov.uk
authors.forename: Luke P
authors.initials: LP
authors.lastname: Randall
authors.forename: Deborah J
authors.initials: DJ
authors.lastname: Eaves
authors.forename: Sue W
authors.initials: SW
authors.lastname: Cooles
authors.forename: V
authors.initials: V
authors.lastname: Ricci
authors.forename: Antony
authors.initials: A
authors.lastname: Buckley
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Woodward
authors.forename: Laura J V
authors.initials: LJ
authors.lastname: Piddock
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000627:therapeutic use*; D002645:Chickens; D027081:DNA Gyrase / Q000235:genetics*; D024901:Drug Resistance, Multiple, Bacterial / Q000187:drug effects / Q000235:genetics; D000077422:Enrofloxacin; D024841:Fluoroquinolones / Q000627:therapeutic use*; D015870:Gene Expression / Q000187:drug effects*; D005798:Genes, Bacterial; D008826:Microbial Sensitivity Tests; D009154:Mutation; D012481:Salmonella Infections, Animal / Q000188:drug therapy* / Q000382:microbiology; D012486:Salmonella typhimurium / Q000187:drug effects* / Q000235:genetics
chemical_list: D000900:Anti-Bacterial Agents; D024841:Fluoroquinolones; D000077422:Enrofloxacin; D027081:DNA Gyrase