Colistin is considered a last resort antibiotic to treat certain multidrug-resistant bacterial infections in humans. Before its classification as an antibiotic of last resort, there was a worldwide use of colistin in both human medicine and animal husbandry; therefore, certain bacteria carrying resistance have emerged.
In Canada, veterinarian should avoid to prescribe colistin, except in rare cases and with justification and certain precautions. Researchers in Canada have identified the presence of the mcr genes, which are responsible for conferring the ability to resist colistin, in certain bacteria (E. coli) isolated from beef or human patients.
However, a Quebec team, leading a project in 2017 on the possibility of a link between animal welfare and the microbiota of pigs (the digestive bacterial flora), highlighted that a commercial farm that does not use colistin had pigs containing genes (mcr-1 and mcr-2) in their microbiota. The study does not indicate whether the genes detected were in the same bacteria or in different bacterial species. One should note that in the USA, another team had also recently carried out the same type of detection in pigs without the use of colistin.
What is the source of animal contamination? The study does not determine it. The authors indicate several potential sources of acquisition of resistant bacteria:
- during the implantation of the microbiota, from maternity;
- through ingestion of contaminated food;
- by selection pressure. As the pig breeding followed within the framework of this study, had used other antibiotics during the period tested: including chlorotetracycline (in post-weaning), it is therefore possible that this antibiotic has favored a selection pressure favorable to bacteria carrying the mcr genes.
- breathing the contaminated air of buildings. Indeed, during previous work on the exposure of pig breeders to aerosol contaminants, Caroline Duchaine's team identified that 60% of air samples from finishing pig buildings in Canada are positive for the mcr-1 gene. In addition, the nasopharyngeal flora of some workers in the pork industry contains these resistance genes.
In Canada, there is a governmental program linking human and veterinary medicine to monitor antibiotic resistance. This Canadian Integrated Antimicrobial Resistance Surveillance Program (CIPARS) tests the resistance patterns of only certain bacteria from hog farms, E. coli and Salmonella, and not all of the bacteria that make up the animal's microbiota. This explains why CIPARS did not detect this, if these genes are not associated with these two bacteria (E. coli and Salmonella). From this farm, 11.4% of pigs sent to the slaughter carried one of the mcr genes in their microbiota, which could pose a threat of transmission of this colistin resistance gene to consumers. However, the risk of exposure appears higher for pig breeders and workers who undergo aerosol exposure and through direct contact with stool.
First identification of mcr-1/mcr-2 genes in the fecal microbiota of Canadian commercial pigs during the growing and finishing period. Rhouma M, Thériault W, Rabhi N, Duchaine C, Quessy S, Fravalo P. Vet Med (Auckl). 2019 Jul 4;10:65-67. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6613599/#CIT0010
Prevalence of mcr-1 in the Cecal Contents of Food Animals in the United States.
Meinersmann RJ, Ladely SR, Plumblee JR, Cook KL, Thacker E. Antimicrob Agents Chemother. 2017 Feb; 61(2):. https://www.ncbi.nlm.nih.gov/pubmed/27855065
Quantification of airborne dust, endotoxins, human pathogens and antibiotic and metal resistance genes in Eastern Canadian swine confinement buildings. Pilote J, Létourneau V, Girard M, Duchaine C. Aerobiologia. 2019;35(2):283–296. https://pubag.nal.usda.gov/catalog/6431499