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Applied and Environmental Microbiology, October 2007, p. 5982-5989, Vol. 73, No. 19
0099-2240/07/$08.00+0     doi:10.1128/AEM.00709-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Influence of Antibiotic Selection on Genetic Composition of Escherichia coli Populations from Conventional and Organic Dairy Farms{triangledown} ,{dagger}

Seth T. Walk,1 Janice M. Mladonicky,1 Jaclyn A. Middleton,1 Anthony J. Heidt,1 Julie R. Cunningham,1 Paul Bartlett,2 Kenji Sato,3 and Thomas S. Whittam1*

Microbial Evolution Laboratory, National Food Safety and Toxicology Center, Michigan State University, East Lansing, Michigan 48824,1 Department of Large Animal Clinical Sciences, National Food Safety and Toxicology Center, Michigan State University, East Lansing, Michigan 48824,2 Department of Veterinary Sciences, University of Wyoming, Laramie, Wyoming 820703

Received 28 March 2007/ Accepted 30 July 2007

The widespread agricultural use of antimicrobials has long been considered a crucial influence on the prevalence of resistant genes and bacterial strains. It has been suggested that antibiotic applications in agricultural settings are a driving force for the development of antimicrobial resistance, and epidemiologic evidence supports the view that there is a direct link between resistant human pathogens, retail produce, farm animals, and farm environments. Despite such concerns, little is understood about the population processes underlying the emergence and spread of antibiotic resistance and the reversibility of resistance when antibiotic selective pressure is removed. In this study, hierarchical log-linear modeling was used to assess the association between farm type (conventional versus organic), age of cattle (calf versus cow), bacterial phenotype (resistant versus susceptible), and the genetic composition of Escherichia coli populations (E. coli Reference Collection [ECOR] phylogroup A, B1, B2, or D) among 678 susceptible and resistant strains from a previously published study of 60 matched dairy farms (30 conventional and 30 organic) in Wisconsin. The analysis provides evidence for clonal resistance (ampicillin resistance) and genetic hitchhiking (tetracycline resistance [Tetr]), estimated the rate of compositional change from conventional farming to organic farming (mean, 8 years; range, 3 to 15 years), and discovered a significant association between low multidrug resistance, organic farms, and strains of the numerically dominant phylogroup B1. These data suggest that organic farming practices not only change the frequency of resistant strains but also impact the overall population genetic composition of the resident E. coli flora. In addition, the results support the hypothesis that the current prevalence of Tetr loci on dairy farms has little to do with the use of this antibiotic.

* Corresponding author. Mailing address: Microbial Evolution Laboratory, 165 Food Safety and Toxicology Building, Michigan State University, East Lansing, MI 48824. Phone: (517) 432-3100, ext. 178. Fax: (517) 432-2310. E-mail: whittam{at}msu.edu

{triangledown} Published ahead of print on 17 August 2007.

{dagger} Supplemental material for this article may be found at http://aem.asm.org/.

Applied and Environmental Microbiology, October 2007, p. 5982-5989, Vol. 73, No. 19
0099-2240/07/$08.00+0     doi:10.1128/AEM.00709-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Bibbal, D., Dupouy, V., Prere, M. F., Toutain, P. L., Bousquet-Melou, A. (2009). Relatedness of Escherichia coli Strains with Different Susceptibility Phenotypes Isolated from Swine Feces during Ampicillin Treatment. Appl. Environ. Microbiol. 75: 2999-3006 [Abstract] [Full Text]  


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