Abstract
The use of antibiotics in animal husbandry contributes to the worldwide problem of increasing antibiotic resistance in animal and human pathogens. Intensive animal production is considered an important source of antibiotic resistance genes released to the environment, while the contribution of smaller farms remains to be evaluated. Here we monitor the spread of tetracycline resistance (TC-r) genes at a middle-size conventional dairy farm, where chlortetracycline (CTC, as intrauterine suppository) is prophylactically used after each calving. Our study has shown that animals at the farm acquired the TC-r genes in their early age (1–2 weeks), likely due to colonization with TC-resistant bacteria from their mothers and/or the farm environment. The relative abundance of the TC-r genes tet(W), tet(Q), and tet(M) in fresh excrements of calves was about 1–2 orders of magnitude higher compared to heifers and dairy cows, possibly due to the presence of antibiotic residues in milk fed to calves. The occurrence and abundance of TC-r genes in fresh excrements of heifers and adult cows remained unaffected by intrauterine CTC applications, with tet(O), tet(Q), and tet(W) representing a “core TC-resistome” of the farm, and tet(A), tet(M), tet(Y), and tet(X) occurring occasionally. The genes tet(A), tet(M), tet(Y), and tet(X) were shown to be respectively harbored by Shigella, Lactobacillus and Clostridium, Acinetobacter, and Wautersiella. Soil in the farm proximity, as well as field soil to which manure from the farm was applied, was contaminated with TC-r genes occurring in the farm, and some of the TC-r genes persisted in the field over 3 months following the manure application. Concluding, our study shows that antibiotic resistance genes may be a stable part of the intestinal metagenome of cattle even if antibiotics are not used for growth stimulation, and that smaller dairy farms may also contribute to environmental pollution with antibiotic resistance genes.
Highlights
Agricultural use of antibiotics contributes to the spread of antibiotic resistance genes that may accumulate in human pathogens, threatening the treatment of infectious diseases (Smith et al, 2005; Forsberg et al, 2012; Durso and Cook, 2014; Jechalke et al, 2014)
The occurrence of the tetracycline resistance (TC-r) genes in calves was not related to their mothers, i.e., the TC-r genes often occurred in calves they were not detected in their mothers
This study has shown that TC-r genes were widespread at the studied dairy farm
Summary
Agricultural use of antibiotics contributes to the spread of antibiotic resistance genes that may accumulate in human pathogens, threatening the treatment of infectious diseases (Smith et al, 2005; Forsberg et al, 2012; Durso and Cook, 2014; Jechalke et al, 2014). The same classes of antibiotics as those used in human medicine are administrated to farm animals for disease treatment and prevention, with tetracyclines and beta-lactams being among the most commonly. Antibiotics entering animal gastrointestinal tracts represent a selection pressure toward antibiotic resistance, and antibiotic resistance genes seem to be a common part of the intestinal metagenome of farm animals (Durso et al, 2011; Lamendella et al, 2011; Wichmann et al, 2014). Antibiotic resistance genes may be shared between animal, soil and human bacteria via horizontal gene transfer (Kobashi et al, 2007; Forsberg et al, 2012) and the increasing contamination of soil with antibiotic resistance genes may, contribute to the worldwide problem of the increasing antibiotic resistance and multiresistance
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
