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Applied and Environmental Microbiology, January 2007, p. 203-210, Vol. 73, No. 1
0099-2240/07/$08.00+0     doi:10.1128/AEM.01482-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Sources of Variation in the Ampicillin-Resistant Escherichia coli Concentration in the Feces of Organic Broiler Chickens{triangledown}

E. J. Pleydell,1,2* P. E. Brown,3 M. J. Woodward,1 R. H. Davies,1 and N. P. French2

Department of Food and Environmental Safety, Veterinary Laboratories Agency, Weybridge, United Kingdom,1 Institute of Veterinary, Animal, and Biomedical Sciences, Massey University, Palmerston North, New Zealand,2 Division of Preventative Oncology, Cancer Care Ontario, and Department of Public Health Services, University of Toronto, Ontario, Canada3

Received 28 June 2006/ Accepted 27 October 2006

Currently, there are limited published data for the population dynamics of antimicrobial-resistant commensal bacteria. This study was designed to evaluate both the proportions of the Escherichia coli populations that are resistant to ampicillin at the level of the individual chicken on commercial broiler farms and the feasibility of obtaining repeated measures of fecal E. coli concentrations. Short-term temporal variation in the concentration of fecal E. coli was investigated, and a preliminary assessment was made of potential factors involved in the shedding of high numbers of ampicillin-resistant E. coli by growing birds in the absence of the use of antimicrobial drugs. Multilevel linear regression modeling revealed that the largest component of random variation in log-transformed fecal E. coli concentrations was seen between sampling occasions for individual birds. The incorporation of fixed effects into the model demonstrated that the older, heavier birds in the study were significantly more likely (P = 0.0003) to shed higher numbers of ampicillin-resistant E. coli. This association between increasing weight and high shedding was not seen for the total fecal E. coli population (P = 0.71). This implies that, in the absence of the administration of antimicrobial drugs, the proportion of fecal E. coli that was resistant to ampicillin increased as the birds grew. This study has shown that it is possible to collect quantitative microbiological data on broiler farms and that such data could make valuable contributions to risk assessments concerning the transfer of resistant bacteria between animal and human populations.

* Corresponding author. Mailing address: Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand. Phone: 64 6 350 5270. Fax: 64 6 350 5716. E-mail: e.j.pleydell{at}massey.ac.nz.

{triangledown} Published ahead of print on 3 November 2006.

Applied and Environmental Microbiology, January 2007, p. 203-210, Vol. 73, No. 1
0099-2240/07/$08.00+0     doi:10.1128/AEM.01482-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.





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