Mohamed Rhouma,1 William Thériault,1 Nassima Rabhi,1 Caroline Duchaine,2 Sylvain Quessy,1 Philippe Fravalo11Department of Pathology and Microbiology, Faculty of Veterinary Medicine, Université de Montréal, Saint-Hyacinthe, QC J2S 2M2, Canada; 2Department of Biochemistry, Microbiology and Bioinformatics, Faculty of Science and Engineering, Université Laval, Québec, QC G1V 4G5, Canada Colistin is considered one of the last-resort antibiotics used for the treatment of infections caused by multidrug-resistant Gram-negative bacteria (MDR-GNB) in humans.1 This antibiotic is classified by the World Health Organization, among the category of the Highest Priority Critically Important Antimicrobials for human medicine.2 In this regard, the global dissemination of plasmid-mediated colistin resistance, conferred by themobile colistin resistance (mcr) genes, has raised serious concern among physicians and scientists worldwide.2 On the other hand, colistin has been widely used in food animals in many countries either for disease therapy, prophylaxis or for growth promotion.3 Many studies reported a high prevalence of mcr genes on bacterial isolates from animals' origin compared to humans, and consequently animals, particularly pig production, were pointed out as the greatest cause of bacterial colistin resistance spread.1 In pigs, mcr genes were isolated mainly from colistin resistant E. coli, Salmonella and Klebsiella isolates, with mcr-1 gene being themost frequently identified gene among the eight types of mcr genes currently described.1 Among other Gram-negative bacteria (GNB) of pig origin, mcr genes were identified in Moraxella spp.,4 and in Aeromonas hydrophila,5 enforcing the interest to consider the mcr presence at the microbiota level. In Canada, although colistin is not approved for use in veterinarymedicine, it was sometimes used in pigs according to special precautions for the oral treatment of digestive infections caused by GNB.1 Nevertheless, mcr-1 gene has been identified in E. coli isolated from humans, as well as from lean ground beef in Canada.6 However, to the best of our knowledge, mcr genes have never been identified in animals at the farm level in Canada. Here we report he screening of fecal samples obtained from one Canadian commercial pig farm, to determine the prevalence over the growing period of mcr-1 and mcr-2 genes in the fecal microbiota of pigs. Colistin has never been used in this farm. However, tiamulin and chlortetracycline were used in the post-weaning period and salinomycin as well as narasin were used during the growing phase.