Associations between antimicrobial resistance genes in fecal generic Escherichia coli isolates in western Canadian cow-calf herds

Dept. of Large Animal Clinical Sciences, Western College of Veterinary Medicine, Saskatoon, Saskatchewan, Canada; Département de Pathologie et Microbiologie, Faculté de Médecine Vétérinaire, Université de Montréal, Saint-Hyacinthe, Québec, Canada; Laboratory for Foodborne Zoonoses, Public Health Agency of Canada, Guelph, Ontario and Saskatoon, Saskatchewan, Canada; Dept. of Pathobiology, University of Guelph, Guelph, Ontario, Canada

^* To whom correspondence should be addressed. Email: sheryl.gow{at}usask.ca.

	Abstract

The objective of this study was to examine associations among the genetic determinants of antimicrobial resistance (AMR) in 207 fecal generic E. coli isolates obtained from 77 cow-calf herds in western Canada. Resistance determinants for 23 genes in 6 different antimicrobial families were assessed using DNA hybridization and PCR. The most common resistance genes in the study sample (207 isolates) were sul2 (48.3%), tet(B) (45.4%) and ant(3'')-Ia (aadA1) (19.3%). There were several statistically significant associations detected between the resistance genes examined. The strongest associations were observed between chloramphenicol (catI) and trimethoprim (dhfrI) (OR=214, P=0.0001), sulfonamide (sul1) and chloramphenicol (catI) (OR=96.9, P=0.0001), streptomycin (ant(3'')-Ia (aadA1)) and trimethoprim (dhfrI) (OR=96.2, P=0.0001), sulfonamide (sul1) and streptomycin (ant(3'')-Ia (aadA1) (OR=79.3, P=0.0001), and tetracycline (tet(B)) and sulfonamides (sul2) (OR=25.7, P=0.0001). At least one of the resistance genes from each family of antimicrobials examined in this study was associated with the two aminoglycoside genes, ant(3'')-Ia (aadA1) and aph(3')-Ia. The multiple, strong associations between genes and the diverse nature of the associations described in this study demonstrated the complexity of resistance gene selection in cow-calf herds and should be considered when planning AMR control in cow-calf operations.