The equine hindgut is a potential source of antimicrobial resistance genes (ARGs) that impact both human and animal health. In this study, we screened equine faecal specimens for the presence of class 1 integrons and associated ARGs. We hypothesised that domestic horses with greater human exposure and veterinary intervention would contain higher levels of ARGs and associated mobile genetic elements such as integrons. Faecal samples were collected from horses on a farm (n=11) and from wild brumbies (n=11). Antibiotic-resistant bacteria were isolated by dilution-plating onto a range of antimicrobials, then colonies were screened for the class 1 integron gene inti1. IntI1 genes, associated gene cassettes and the 16S rDNA of PCR-positive bacteria were sequenced.
Fourteen isolates (10 Gram positives, 4 Gram negatives) from antibiotic agar plates contained class 1 integrons. These were identified by 16S sequencing as Kocuria palustris, Arthrobacter citreus, Arthrobacter luteolus, Microbacterium esteraromaticum, Micrococcus terreus, Microbacterium aurum, Rhodococcus coprophilus, Escherichia hermanii, Leclercia adecarboxylata, Pantoea sp., and Pseudomonas sp. The intI1-positive isolates were obtained on streptomycin, sulfamethoxazole, tetracycline, and trimethoprim plates; they all had conserved 3’ sections (qacEdelta1-sul1), and they yielded six distinct gene cassette array types: aadA2; aadA9; dfrA5; dfrA16; dfrA1-aadA6, or no cassettes.
The prevalence of Gram-positive bacteria among the class 1 integron positive isolates was unexpected. Further analysis of these isolates by complete genome sequencing is underway.