In 2015, the scientific community discovered mcr-1, a new bacterial gene that is distributed throughout the world and confers resistance to colistin, a last-resort antibiotic in human medicine. 2015 was also the year in which a colistin-resistant strain of the Escherichia coli bacterium was first found. It was detected in a cattle slaughterhouse in Spain, through a monitoring programme for antimicrobial resistance in zoonotic and commensal bacteria run by the country’s Ministry of Agriculture. Two years later, the farm on which the resistant strain originated was identified and a team of researchers from the Institute of Agrifood Research and Technology (IRTA) visited it to analyse faecal samples from the animals and the farmer. Carried out by the Molecular Genetics Veterinary Service (SVGM) of the Autonomous University of Barcelona (UAB), and recently published in the journal Antibiotics, the analysis and sequencing of the samples have shown that the farmer and the calves and pigs studied all had E. coli bacteria with the same resistance gene.
In this case, the experts concluded that the resistance gene had been transmitted from the animals to the human, as the calves and pigs had been treated with colistin whereas the farmer had not. “Bacteria are microorganisms with two types of genetic material, chromosomal DNA and plasmids,” explains the study’s leader, Lourdes Migura, a researcher involved in IRTA’s Animal Health programme. “Many antibiotic resistance genes are found on plasmids. When two bacteria come into contact, the resistance ‘superpower’ can be transmitted from one to the other if they exchange plasmids. That is how the mcr-1 colistin resistance gene is passed on from bacterium to bacterium.”
According to the experts, the farmer could have acquired the colistin resistance of the animals’ bacteria through direct contact with the animals, their excrement or contaminated work tools. “This is a case of horizontal transfer of resistance genes through plasmid exchange,” says SVGM researcher Joaquim Viñes. “The study clearly shows that animals can be reservoirs of such genes and transmit them to humans, so it is important that every hygiene measure possible be taken on livestock farms.”
On farms, colistin was used regularly until 2016, mainly for treating infections such as post-weaning diarrhoea, but the discovery of colistin-resistant bacteria is indicative of the antibiotic’s overuse. In human medicine, it is now only used in hospitals as a last resort in serious cases, when conventional drugs have failed to halt bacterial infections. “Having resistant E. coli bacteria is not directly dangerous in itself, but they are opportunistic bacteria,” remarks Migura. “That means that if something else causes a person with resistant E. coli to become seriously ill, the opportunistic bacteria could take advantage of the body’s weakened defences to infect it, resulting in a patient who would be very difficult to treat,” she explains. “If that person needed to be admitted to hospital, there would also be the risk of them passing on the bacteria or the bacteria’s plasmid to other people,” she adds.
The use of antibiotics in human and veterinary medicine has led to the emergence of resistant bacteria, the importance of which in terms of human health cannot be overstated. In Spain and worldwide, there are various initiatives for reducing antibiotic use and inhibiting the appearance of ‘superbugs’, given the global public health threat entailed. Furthermore, IRTA is working to reduce the use of antibiotics in animal production as part of its Strategic Plan for the next three years.
Reference of the article:
Viñes, J., Cuscó, A., Napp, S., Alvarez, J., Saez-Llorente, J. L., Rosàs-Rodoreda, M., … & Migura-Garcia, L. (2021). Transmission of Similar Mcr-1 Carrying Plasmids among Different Escherichia coli Lineages Isolated from Livestock and the Farmer. Antibiotics, 10(3), 313. https://doi.org/10.3390/antibiotics10030313
