ABSTRACT Antimicrobial resistance in bacteria is a threat to both human and animal health. We aimed to understand the impact of domestication and antimicrobial treatment on the types and numbers of resistant bacteria, antibiotic resistance genes (ARGs), and class 1 integrons (C1I) in the equine gut microbiome. Antibiotic-resistant faecal bacteria were isolated from wild horses, healthy farm horses, and horses undergoing veterinary treatment, and isolates (9,083 colonies) were screened by PCR for C1I; these were found at frequencies of 9.8% (vet horses), 0.31% (farm horses), and 0.05% (wild horses). A collection of 71 unique C1I + isolates (17 Actinobacteria and 54 Proteobacteria) was subjected to resistance profiling and genome sequencing. Farm horses yielded mostly C1I + Actinobacteria ( Rhodococcus, Micrococcus, Microbacterium, Arthrobacter, Glutamibacter, Kocuria) , while vet horses primarily gave C1I + Proteobacteria ( Escherichia, Klebsiella, Enterobacter, Pantoea, Acinetobacter, Leclercia, Ochrobactrum) ; the vet isolates had more extensive resistance and stronger P C promoters in the C1Is. All integrons in Actinobacteria were flanked by copies of IS 6100 , except in Micrococcus , where a novel IS 5 family element (IS Mcte1 ) was implicated in mobilization. In the Proteobacteria, C1I’s were predominantly associated with IS 26 , and also IS 1 , Tn 21 , Tn 1721 , Tn 512 , and a putative formaldehyde-resistance transposon (Tn 7489 ). Several large C1I-containing plasmid contigs were retrieved; two of these (plasmid types Y and F) also had extensive sets of metal resistance genes, including a novel copper-resistance transposon (Tn 7519 ). Both veterinary treatment and domestication increase the frequency of C1I’s in equine gut microflora, and each of these anthropogenic factors selects for a distinct group of integron-containing bacteria. IMPORTANCE There is increasing acknowledgement that a ‘One Health’ approach is required to tackle the growing problem of antimicrobial resistance. This requires that the issue is examined from not only the perspective of human medicine, but also includes consideration of the roles of antimicrobials in veterinary medicine and agriculture, and recognises the importance of other ecological compartments in the dissemination of ARGs and mobile genetic elements such as C1I. We have shown that domestication and veterinary treatment increase the frequency of occurrence of C1I’s in the equine gut microflora, and that in healthy farm horses, the C1I are unexpectedly found in Actinobacteria, while in horses receiving antimicrobial veterinary treatments, a taxonomic shift occurs, and the more typical integron-containing Proteobacteria are found. We identified several new mobile genetic elements (plasmids, IS and transposons) on genomic contigs from the integron-containing equine bacteria.

Integrons, plasmids, and resistance genes in equine faecal bacteria