JAC Advance Access published online on November 12, 2007
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkm415
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Fitness and dissemination of disinfectant-selected multiple-antibiotic-resistant (MAR) strains of Salmonella enterica serovar Typhimurium in chickens
1 Department of Food and Environmental Safety, Veterinary Laboratories Agency, New Haw, Surrey KT15 3NB, UK 2 Faculty of Medical and Veterinary Sciences, University of Bristol, Langford House, Langford, Bristol BS40 5DU, UK 3 Antimicrobial Agents Research Group, Division of Immunity and Infection, University of Birmingham, Birmingham B15 2TT, UK
Received 17 July 2007; returned 28 September 2007; revised 3 October 2007; accepted 8 October 2007
* Corresponding author. Tel: +44-1932-357906; Fax: +44-1932-357595; E-mail: l.randall{at}vla.defra.gsi.gov.uk
Objectives: The aims of this study were to determine whether strains of Salmonella enterica serovar Typhimurium which had acquired low-level multiple antibiotic resistance (MAR) through repeated exposure to farm disinfectants were able to colonize and transmit between chicks as easily as the parent strain and, if such strains were less susceptible to fluoroquinolones, would high-level resistance be selected after fluoroquinolone treatment.
Methods: Two mutants were compared with the isogenic parent. In the first experiment, day-old chicks were co-infected with both the parent and a mutant to determine their relative fitness. In the second experiment, parent and mutant strains (in separate groups of chicks) were assessed for their ability to transmit from infected (contact) to non-infected (naive) birds and with respect to their susceptibility to fluoroquinolone treatment. Birds were regularly monitored for the presence of Salmonella in caecal contents. Replica plating was used to monitor for the selection of antibiotic-resistant strains.
Results: The parent strain was shown to be significantly fitter than the two mutants and was more rapidly disseminated to naive birds. Antibiotic treatment did not preferentially select for the two mutants or for resistant strains.
Conclusions: The disinfectant-exposed strains, although MAR, were less fit, less able to disseminate than the parent strain and were not preferentially selected by therapeutic antibiotic treatment. As such, these strains are unlikely to present a greater problem than other salmonellae in chickens.
Key Words: fluoroquinolones , enrofloxacin , birds , microbial pathogenicity , multidrug exporter