AEM Accepts, published online ahead of print on 23 May 2008

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Appl. Environ. Microbiol. doi:10.1128/AEM.00489-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Effect of subtherapeutic administration of antibiotics on the prevalence of antibiotic-resistant Escherichia coli in feedlot cattle

T. W. Alexander, L. J. Yanke, E. Topp, M. E. Olson, R. R. Read, D. W. Morck, and T. A. McAllister^*

Agriculture and Agri-Food Canada Research Centre, Lethbridge, Alberta T1J 4B1; London Ontario, Canada N5V 4T3; Bow Valley Research, Calgary, Alberta S9H 3X2; Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada T2N 4N1; Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada T2N 1N4

^* To whom correspondence should be addressed. Email: mcallistert{at}agr.gc.ca.

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 + sulfamethazine (TET-SUL); chlortetracycline (TET); virginiamycin (VIR); monensin (MON); tylosin (TYL), 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 prevalence of steers shedding TET - and AMP - resistant E. coli was >40 and <30 percent, respectively. Inclusion of TET-SUL in the diet increased the prevalence of steers shedding TET and AMP resistant E. coli and the percent 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 those fed grain-based, as 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 of AMP, gentamicin, or TET, respectively. Additionally, 131 (1.8%) and 143 (2.0%) isolates exhibited potential resistance to extended-spectrum -lactamases as indicated by either ceftazidime or cefpodoxime resistance. No isolates were resistant to ciprofloxacin. The findings of this study indicated that subtherapeutic administration of tetracycline in combination with sulfamethazine increased the prevalence of tetracycline- and ampicillin- resistant E. coli in cattle. However, resistance to antibiotics may be related to additional environmental factors such as diet.

This article has been cited by other articles:

• Alexander, T. W., Reuter, T., Sharma, R., Yanke, L. J., Topp, E., McAllister, T. A. (2009). Longitudinal Characterization of Resistant Escherichia coli in Fecal Deposits from Cattle Fed Subtherapeutic Levels of Antimicrobials. Appl. Environ. Microbiol. 75: 7125-7134 [Abstract] [Full Text]