Previous Article | Next Article 
Applied and Environmental Microbiology, July 2008, p. 4405-4416, Vol. 74, No. 14
0099-2240/08/$08.00+0 doi:10.1128/AEM.00489-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Effect of Subtherapeutic Administration of Antibiotics on the Prevalence of Antibiotic-Resistant Escherichia coli Bacteria in Feedlot Cattle
T. W. Alexander,1
L. J. Yanke,1
E. Topp,2
M. E. Olson,3
R. R. Read,4
D. W. Morck,5 and
T. A. McAllister1*
Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, Alberta T1J 4B1, Canada,1 Agriculture and Agri-Food Canada, Southern Crop Protection and Food Research Centre, London, Ontario N5V 4T3, Canada,2 Bow Valley Research, Calgary, Alberta T2N 4G3, Canada,3 Faculty of Medicine, University of Calgary, Calgary, Alberta T2N 4N1, Canada,4 Department of Biological Sciences, University of Calgary, Calgary, Alberta T2N 1N4, Canada5
Received 28 February 2008/ Accepted 13 May 2008
Antibiotic-resistant Escherichia coli in 300 feedlot steers receiving subtherapeutic levels of antibiotics was investigated through the collection of 3,300 fecal samples over a 314-day period. Antibiotics were selected based on the commonality of use in the industry and included chlortetracycline plus sulfamethazine (TET-SUL), chlortetracycline (TET), virginiamycin, monensin, tylosin, or no antibiotic supplementation (control). Steers were initially fed a barley silage-based diet, followed by transition to a barley grain-based diet. Despite not being administered antibiotics prior to arrival at the feedlot, the prevalences of steers shedding TET- and ampicillin (AMP)-resistant E. coli were >40 and <30%, respectively. Inclusion of TET-SUL in the diet increased the prevalence of steers shedding TET- and AMP-resistant E. coli and the percentage of TET- and AMP-resistant E. coli in the total generic E. coli population. Irrespective of treatment, the prevalence of steers shedding TET-resistant E. coli was higher in animals fed grain-based compared to silage-based diets. All steers shed TET-resistant E. coli at least once during the experiment. A total of 7,184 isolates were analyzed for MIC of antibiotics. Across antibiotic treatments, 1,009 (13.9%), 7 (0.1%), and 3,413 (47.1%) E. coli isolates were resistant to AMP, gentamicin, or TET, respectively. In addition, 131 (1.8%) and 143 (2.0%) isolates exhibited potential resistance to extended-spectrum β-lactamases, as indicated by either ceftazidime or cefpodoxime resistance. No isolates were resistant to ciprofloxacin. The findings of the present study indicated that subtherapeutic administration of tetracycline in combination with sulfamethazine increased the prevalence of tetracycline- and AMP-resistant E. coli in cattle. However, resistance to antibiotics may be related to additional environmental factors such as diet.
* Corresponding author. Mailing address: Agriculture and Agri-Food Canada Research Centre, 5403 1st Ave. South, P.O. Box 3000, Lethbridge, Alberta T1J 4B1, Canada. Phone: (403) 317-2240. Fax: (403) 382-3156. E-mail: mcallistert{at}agr.gc.ca
Published ahead of print on 23 May 2008.
Applied and Environmental Microbiology, July 2008, p. 4405-4416, Vol. 74, No. 14
0099-2240/08/$08.00+0 doi:10.1128/AEM.00489-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»