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Applied and Environmental Microbiology, November 2005, p. 6680-6688, Vol. 71, No. 11
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.11.6680-6688.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Molecular Epidemiology of Antimicrobial-Resistant Commensal Escherichia coli Strains in a Cohort of Newborn Calves

Deborah V. Hoyle,1,2 Catherine M. Yates,2 Margo E. Chase-Topping,1* Esther J. Turner,2 Sarah E. Davies,2 J. Chris Low,3 George J. Gunn,3 Mark E. J. Woolhouse,1 and Sebastian G. B. Amyes2

Centre for Tropical Veterinary Medicine, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Veterinary Centre, Roslin, Midlothian EH25 9RG, United Kingdom,1 Medical Microbiology, University of Edinburgh, Medical School, Teviot Place, Edinburgh EH8 9AG, United Kingdom,2 Scottish Agricultural College, Animal Health Group, Research Division, West Mains Road, Edinburgh EH9 3JG, United Kingdom3

Received 17 December 2004/ Accepted 23 June 2005

Pulsed-field gel electrophoresis (PFGE) was used to investigate the dissemination and diversity of ampicillin-resistant (Ampr) and nalidixic acid-resistant (Nalr) commensal Escherichia coli strains in a cohort of 48 newborn calves. Calves were sampled weekly from birth for up to 21 weeks and a single resistant isolate selected from positive samples for genotyping and further phenotypic characterization. The Ampr population showed the greatest diversity, with a total of 56 different genotype patterns identified, of which 5 predominated, while the Nalr population appeared to be largely clonal, with over 97% of isolates belonging to just two different PFGE patterns. Distinct temporal trends were identified in the distribution of several Ampr genotypes across the cohort, with certain patterns predominating at different points in the study. Cumulative recognition of new Ampr genotypes within the cohort was biphasic, with a turning point coinciding with the housing of the cohort midway through the study, suggesting that colonizing strains were from an environmental source on the farm. Multiply resistant isolates dominated the collection, with >95% of isolates showing resistance to at least two additional antimicrobials. Carriage of resistance to streptomycin, sulfamethoxazole, and tetracycline was the most common combination, found across several different genotypes, suggesting the possible spread of a common resistance element across multiple strains. The proportion of Ampr isolates carrying sulfamethoxazole resistance increased significantly over the study period (P < 0.05), coinciding with a decline in the most common genotype pattern. These data indicate that calves were colonized by a succession of multiply resistant strains, with a probable environmental source, that disseminated through the cohort over time.

* Corresponding author. Mailing address: Centre for Tropical Veterinary Medicine, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Veterinary Centre, Roslin, Midlothian, Scotland EH25 9RG, United Kingdom. Phone: 44 (0) 131 650 7263. Fax: 44 (0) 131 651 3903. E-mail: Margo.Chase{at}ed.ac.uk.

Applied and Environmental Microbiology, November 2005, p. 6680-6688, Vol. 71, No. 11
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.11.6680-6688.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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