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Applied and Environmental Microbiology, March 2009, p. 1373-1380, Vol. 75, No. 5
0099-2240/09/$08.00+0     doi:10.1128/AEM.01253-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Associations between Antimicrobial Resistance Phenotypes, Antimicrobial Resistance Genes, and Virulence Genes of Fecal Escherichia coli Isolates from Healthy Grow-Finish Pigs {triangledown}

Leigh B. Rosengren,1* Cheryl L. Waldner,1 and Richard J. Reid-Smith2,3

Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, 52 Campus Drive, Saskatoon, Saskatchewan S7N 5B4, Canada,1 Antimicrobial Resistance Surveillance Unit, Laboratory for Foodborne Zoonoses, Public Health Agency of Canada, 160 Research Lane, Unit 103, Guelph, Ontario N1G 5B2, Canada,2 Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, Ontario N1G 2W1, Canada3

Received 5 June 2008/ Accepted 2 January 2009

Escherichia coli often carries linked antimicrobial resistance genes on transmissible genetic elements. Through coselection, antimicrobial use may select for unrelated but linked resistance or virulence genes. This study used unconditional statistical associations to investigate the relationships between antimicrobial resistance phenotypes and antimicrobial resistance genes in 151 E. coli isolates from healthy pigs. Phenotypic resistance to each drug was significantly associated with phenotypic resistance to at least one other drug, and every association found that the probability of observing the outcome resistance was increased by the presence of the predictor resistance. With one exception, each statistical association that was identified between a pair of resistance genes had a corresponding significant association identified between the phenotypes mediated by those genes. This suggests that associations between resistance phenotypes might predict coselection. If this hypothesis is confirmed, evaluation of the associations between resistance phenotypes could improve our knowledge of coselection dynamics and provide a cost-effective way to evaluate existing data until large-scale genotypic data collection becomes feasible. This could enable policy makers and users of antimicrobials to consider coselection in antimicrobial use decisions. This study also considered the unconditional relationships between resistance and virulence genes in E. coli from healthy pigs (aidA-1, eae, elt, estA, estB, fedA1, stx1, and stx2). Positive statistical associations would suggest that antimicrobial use may select for virulence in bacteria that may contaminate food or cause diarrhea in pigs. Fortunately, the odds of detecting a virulence gene were rarely increased by the presence of an antimicrobial resistance gene. This suggests that on-farm antimicrobial use did not select for the examined virulence factors in E. coli carried by this population of healthy pigs.

* Corresponding author. Mailing address: Box 451, Midale, Saskatchewan S0C 1S0, Canada. Phone: (306) 458-2967. Fax: (306) 458-2267. E-mail: leigh.rosengren{at}vetepi.com

{triangledown} Published ahead of print on 9 January 2009.

Applied and Environmental Microbiology, March 2009, p. 1373-1380, Vol. 75, No. 5
0099-2240/09/$08.00+0     doi:10.1128/AEM.01253-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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