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Appl. Environ. Microbiol. doi:10.1128/AEM.01132-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Effect of antimicrobial dosage regime on Salmonella and Escherichia coli isolated from feeder swine

Centers for Epidemiology and Animal Health, Animal and Plant Health Inspection Service, U.S. Department of Agriculture, Fort Collins, Colorado 80526-8117, USA; Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan 48824, USA; Bacterial Epidemiology and Antimicrobial Resistance Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Athens, Georgia 30605, USA

^* To whom correspondence should be addressed. Email: Bruce.A.Wagner{at}aphis.usda.gov.

	Abstract

A body of evidence exists that suggests that antimicrobial use in food animals leads to resistance in both pathogenic and commensal bacteria. This study focused on the impact of 3 different antimicrobial regimes (low-level continuous, pulse, and no antimicrobial) for 2 antimicrobials (chlortetracycline and tylosin) on the presence of Salmonella spp. and on the prevalence of antimicrobial resistance of both Salmonella spp. and nonspecific Escherichia coli in fecal samples from feeder swine. The prevalence of fecal samples positive for Salmonella spp. significantly decreased between the samples taken at feeder placement compared to samples taken when the animals were close to market weight. Differences in resistance of Salmonella spp. did not appear to be influenced by dosing treatment including the control. Analysis antimicrobial resistance examining both susceptible/resistance and minimum inhibitory concentration outcomes demonstrated that only resistance to cephalothin increased in E. coli under the pulse chlortetracycline treatment. These results suggest that the dosing regimes examined in this study did not lead to an increase in either the prevalence of Salmonella spp or the prevalence of antimicrobial resistance in isolates of Salmonella spp. or E. coli.