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Applied and Environmental Microbiology, October 2005, p. 5865-5872, Vol. 71, No. 10
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.10.5865-5872.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Effects of Orally Administered Tetracycline on the Intestinal Community Structure of Chickens and on tet Determinant Carriage by Commensal Bacteria and Campylobacter jejuni

A. S. Fairchild,1 J. L. Smith,1 U. Idris,1 J. Lu,1 S. Sanchez,2 L. B. Purvis,1 C. Hofacre,1 and M. D. Lee1*

Department of Population Health,1 Athens Diagnostic Laboratory, College of Veterinary Medicine, University of Georgia, Athens, Georgia 306022

Received 2 September 2004/ Accepted 28 April 2005

There is a growing concern that antibiotic usage in animal production has selected for resistant food-borne bacteria. Since tetracyclines are common therapeutic antibiotics used in poultry production, we sought to evaluate the effects of oral administration on the resistance of poultry commensal bacteria and the intestinal bacterial community structure. The diversity indices calculated from terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA amplicons did not indicate significant changes in the cecal bacterial community in response to oxytetracycline. To evaluate its effects on cultivable commensals, Enterococcus spp., Escherichia coli, and Campylobacter spp. were isolated from the cecal droppings of broiler chickens. Enterococcus spp. and E. coli expressed tetracycline MICs of >8 µg/ml and harbored a variety of tet resistance determinants regardless of the tetracycline exposure history of the birds. The enterococcal isolates possessed tetM (61%), tetL (25.4%), and tetK (1.3%), as well as tetO (52.5%), the determinant known to confer a tetracycline resistance phenotype in Campylobacter jejuni. E. coli isolates harbored tetA (32.2%) or tetB (30.5%). Tetracycline MICs remained at <2 µg/ml for Campylobacter isolates before and after tetracycline treatment of the chickens, even though isolates expressing MICs of >16 µg/ml were commonly cultured from flocks that did not receive oxytetracycline. The results imply that complex ecological and genetic factors contribute to the prevalence of antibiotic resistance arising from resistance gene transfer in the production environment.

* Corresponding author. Mailing address: Department of Population Health, The University of Georgia, 953 College Station Rd., Athens, GA 30602-4875. Phone: (706) 583-0797. Fax: (706) 542-5630. E-mail: leem{at}vet.uga.edu.

Applied and Environmental Microbiology, October 2005, p. 5865-5872, Vol. 71, No. 10
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.10.5865-5872.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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