Affiliation:
1. Microbial Evolution Laboratory, National Food Safety and Toxicology Center, Michigan State University, East Lansing, Michigan 48824
2. Department of Large Animal Clinical Sciences, National Food Safety and Toxicology Center, Michigan State University, East Lansing, Michigan 48824
3. Department of Veterinary Sciences, University of Wyoming, Laramie, Wyoming 82070
Abstract
ABSTRACT
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 [Tet
r
]), 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 Tet
r
loci on dairy farms has little to do with the use of this antibiotic.
Publisher
American Society for Microbiology
Subject
Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology