Journal Article

Fluoroquinolone treatment of experimental <i>Salmonella enterica</i> serovar Typhimurium DT104 infections in chickens selects for both <i>gyrA</i> mutations and changes in efflux pump gene expression

Luke P. Randall, Deborah J. Eaves, Sue W. Cooles, V. Ricci, Antony Buckley, Martin J. Woodward and Laura J. V. Piddock

in Journal of Antimicrobial Chemotherapy

Published on behalf of British Society for Antimicrobial Chemotherapy

Volume 56, issue 2, pages 297-306
Published in print August 2005 | ISSN: 0305-7453
Published online June 2005 | e-ISSN: 1460-2091 | DOI: http://dx.doi.org/10.1093/jac/dki189
Fluoroquinolone treatment of experimental Salmonella enterica serovar Typhimurium DT104 infections in chickens selects for both gyrA mutations and changes in efflux pump gene expression

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  • Medical Oncology
  • Critical Care

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Objectives: 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 ∼103 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.

Keywords: MAR; cyclohexane tolerance; quinolone resistance

Journal Article.  7228 words.  Illustrated.

Subjects: Medical Oncology ; Critical Care

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