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Applied and Environmental Microbiology, October 2008, p. 6178-6186, Vol. 74, No. 20
0099-2240/08/$08.00+0 doi:10.1128/AEM.00704-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Diversity and Distribution of Commensal Fecal Escherichia coli Bacteria in Beef Cattle Administered Selected Subtherapeutic Antimicrobials in a Feedlot Setting
,
Ranjana Sharma,1*
Krysty Munns,1
Trevor Alexander,1
Toby Entz,1
Parasto Mirzaagha,1
L. Jay Yanke,1
Michael Mulvey,2
Edward Topp,3 and
Tim McAllister1
Agriculture and Agri-Food Research Centre, Lethbridge, Alberta, Canada T1J 4B1,1 Public Health Agency of Canada, Winnipeg, Manitoba, Canada R3E 3R2,2 Agriculture and Agri-Food Research Centre, London, Ontario, Canada N5V 4T33
Received 25 March 2008/ Accepted 11 August 2008
Escherichia coli strains isolated from fecal samples were screened to examine changes in phenotypic and genotypic characteristics including antimicrobial susceptibility, clonal type, and carriage of resistance determinants. The goal of this 197-day study was to investigate the influence of administration of chlortetracycline alone (T) or in combination with sulfamethazine (TS) on the development of resistance, dissemination of defined strain types, and prevalence of resistance determinants in feedlot cattle. Inherent tetracycline resistance was detected in cattle with no prior antimicrobial exposure. Antimicrobial administration was not found to be essential for the maintenance of inherently ampicillin-resistant and tetracycline-resistant (Tetr) E. coli in control animals; however, higher Tetr E. coli shedding was observed in animals subjected to the two treatments. At day 0, high tetracycline (26.7%), lower sulfamethoxazole-tetracycline (19.2%), and several other resistances were detected, which by the finishing phase (day 197) were restricted to ampicillin-tetracycline (47.5%), tetracycline (31.7%), and ampicillin-tetracycline-sulfamethoxazole (20.8%) from both treated and untreated cattle. Among the determinants, blaTEM1, tet(A), and sul2 were prevalent at days 0 and 197. Further, E. coli from day 0 showed diverse antibiogram profiles and strain types, which by the finishing phase were limited to up to three, irrespective of the treatment. Some genetically identical strains expressed different phenotypes and harbored diverse determinants, indicating that mobile genetic elements contribute to resistance dissemination. This was supported by an increased linked inheritance of ampicillin and tetracycline resistance genes and prevalence of specific strains at day 197. Animals in the cohort shed increasingly similar genotypes by the finishing phase due to animal-to-animal strain transmission. Thus, characterizing inherent resistance and propagation of cohort-specific strains is crucial for determining antimicrobial resistance in cattle.
* Corresponding author. Mailing address: 5403 1st Avenue S, P.O. Box 3000, Lethbridge, AB T1J 4B1, Canada. Phone: (403) 317-2232. Fax: (403) 382-3156. E-mail: sharmar{at}agr.gc.ca
Published ahead of print on 22 August 2008.
Contribution 38707054 from Agriculture and Agri-Food Research Centre, Lethbridge, Alberta, Canada.
Applied and Environmental Microbiology, October 2008, p. 6178-6186, Vol. 74, No. 20
0099-2240/08/$08.00+0 doi:10.1128/AEM.00704-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
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