Influence of Antibiotic Selection on Genetic Composition of Escherichia coli Populations from Conventional and Organic Dairy Farms

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 ( , 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.

Changes in Multidrug Resistance of Enteric Bacteria following an Intervention To Reduce Antimicrobial Resistance in Dairy Calves

“…Calf age was associated with changes in MDR in fecal E. coli isolates, which agrees with results from studies indicating that resistance declines as calves age (5,9,20). Positive associations between MDR and morbidity have been reported in other studies (2,17,18,19) and were confirmed by the positive associations found in this study between resistance and herd-level rates of diarrhea in preweaned calves and cows in the month prior to sample collection.…”

“…MDR levels declined after the initiation of the intervention but later returned to preintervention levels. This pattern has been reported in other studies (10), and investigators found that it takes 8 years for significant shifts in the genetic composition of E. coli after starting organic practices (20), which suggests that more time is necessary to see long-term changes in cattle gut flora on the intervention farms.…”

Prevalence of Antimicrobial Resistance and Transfer of Tetracycline Resistance Genes in Escherichia coli Isolates from Beef Cattle

“…Actually, tet(A) and/or tet(B), encoding efflux mechanisms, has been reported to be the most common tetracycline resistance determinant in E. coli isolates from humans and animals in many countries (12,13,(20)(21)(22). Previous studies conducted in cattle disagree: some have reported that the tet(A) determinant is dominant in E. coli isolates recovered from cattle (23)(24)(25), whereas others found tet(B) to be dominant (26)(27)(28). In the present study, the prevalences of tet(A) and tet(B) were almost equal at 46.5% and 45.1%, respectively, which is consistent with other reports that showed a similar distribution pattern for the tet gene in E. coli isolates recovered from animals (23,29).…”

Relatedness of Escherichia coli Strains with Different Susceptibility Phenotypes Isolated from Swine Feces during Ampicillin Treatment

“…Groups B2 and D, which potentially include virulent extraintestinal strains (5,29,33), were rare. The predominance of group B1 has also been observed in fecal E. coli populations of bovine origin (21,51). In swine, it was shown that most commensal strains belonged to groups A and B1 (7), with occasional predominance of phylogroup A (41,52).…”