Diversity and Distribution of Commensal Fecal Escherichia coli Bacteria in Beef Cattle Administered Selected Subtherapeutic Antimicrobials in a Feedlot Setting

Abstract

Escherichia coli strains isolated from fecal samples were screened to examine changes in phenotypic and genotypic characteristics including antimicrobial susceptibility, clonal type, and carriage of resistance determinants. The goal of this 197-day study was to investigate the influence of administration of chlortetracycline alone (T) or in combination with sulfamethazine (TS) on the development of resistance, dissemination of defined strain types, and prevalence of resistance determinants in feedlot cattle. Inherent tetracycline resistance was detected in cattle with no prior antimicrobial exposure. Antimicrobial administration was not found to be essential for the maintenance of inherently ampicillin-resistant and tetracycline-resistant (Tet r ) E. coli in control animals; however, higher Tet r E. coli shedding was observed in animals subjected to the two treatments. At day 0, high tetracycline (26.7%), lower sulfamethoxazole-tetracycline (19.2%), and several other resistances were detected, which by the finishing phase (day 197) were restricted to ampicillin-tetracycline (47.5%), tetracycline (31.7%), and ampicillin-tetracycline-sulfamethoxazole (20.8%) from both treated and untreated cattle. Among the determinants, bla TEM1 , tet(A), and sul2 were prevalent at days 0 and 197. Further, E. coli from day 0 showed diverse antibiogram profiles and strain types, which by the finishing phase were limited to up to three, irrespective of the treatment. Some genetically identical strains expressed different phenotypes and harbored diverse determinants, indicating that mobile genetic elements contribute to resistance dissemination. This was supported by an increased linked inheritance of ampicillin and tetracycline resistance genes and prevalence of specific strains at day 197. Animals in the cohort shed increasingly similar genotypes by the finishing phase due to animal-to-animal strain transmission. Thus, characterizing inherent resistance and propagation of cohort-specific strains is crucial for determining antimicrobial resistance in cattle.In recent years, the development of antimicrobial resistance (AR) in intensive livestock production has been increasingly reported (17). Consequently, the role of antimicrobial administration and the extent to which it affects the development of resistance in animals are receiving much attention. Antimicrobial usage for livestock can be for therapeutic, prophylactic, metaphylactic, or growth promotion purposes (26). Reportedly, 90% of the antimicrobials used in animal agriculture are for growth promotion and prophylaxis (26) and this widespread use is suggested to be an important contributor to the emergence, selection, and dissemination of antimicrobial-resistant bacteria, as indicated by several recent studies (9, 13, 28, 38). Antimicrobial-resistant bacteria, including Escherichia coli, are frequently isolated from the commensal gut flora of food animals (1, 2), and although the resistance they carry may not be a problem per se, the transfer of resistance elements to zoonotic...

Longitudinal Characterization of Resistant Escherichia coli in Fecal Deposits from Cattle Fed Subtherapeutic Levels of Antimicrobials

“…Similar results have been described for fecal samples that have been collected directly from the rectums of feedlot cattle (1,46) and dairy calves (25) that have not been fed AGP. Given the widespread prevalence of tetracycline resistance (40), presumably these cattle were colonized by Tet r bacteria from other environmental sources, most likely prior to their arrival at the feedlot.…”

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).…”

Food Animals and Antimicrobials: Impacts on Human Health

“…Each animal feeding on an antibiotic becomes a "factory" for the production and subsequent dispersion of antibiotic-resistant bacteria. NTA uses are also clearly linked to the propagation of multidrug resistance (MDR), including resistance against drugs that were never used on the farm (10,52,59,60,92,107,111,132,141,153,154,164). The chronic use of a single antibiotic selects for resistance to multiple structurally unrelated antibiotics via linkage of genes on plasmids and transposons (111,143).…”