Does Antibiotic Resistance Move Through the Environment?

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Retrieved: September 19, 2019, 1:28 am

This article written by Dr. Reynold Bergen, BCRC Science Director, originally appeared in the December 2018 issue of Canadian Cattlemen magazine and is reprinted on the BCRC Blog with permission of the publisher.


Recent columns have talked about antibiotic use in Canadian cow-calf and feedlot operations. Contrary to common misconceptions, antibiotic resistant bacteria are very unlikely to transfer from cattle to beef, evade food safety interventions in the processing plant, survive cooking, and cause an antibiotic resistant infection in a person. But can antibiotic resistant bacteria be transmitted from cattle, through feedlot manure and runoff, across soil, through wetlands, streams and rivers, and reach humans through the environment?

A Beef Science Cluster study led by Dr. Rahat Zaheer and Tim McAllister of Agriculture and Agri-Food Canada (with collaborators from the Public Health Agency of Canada, the University of Calgary’s faculties of medicine and veterinary medicine, University of Guelph, Alberta Agriculture and Feedlot Health Management Services) examined this question.

What they did: This research focused on bacteria called enterococci that can cause infections in humans (e.g. urinary tract, liver and bile duct, heart, surgery wound, and bloodstream infections). Most enterococcal infections can be effectively treated with macrolide antibiotics. This is important because macrolides (products like Draxxin, Zuprevo, Micotil, Tylan, Zactran, etc.) are commonly used in both beef production and human medicine.

Over a two-year period, this team collected samples from feedlots (pen floor fecal samples, collection ponds, stockpiled and composted manure), agricultural soils, wetlands, streams, municipal sewage, packing plants, retail meats and human patients. Advanced lab testing was used to identify the specific types of enterococci and antibiotic resistance patterns in the samples from each location.

What they learned: Different species of enterococci thrive in different environments. For example, one species (E. hirae) accounted for 90% of the enterococci found in fecal samples from cattle but less than 1% of the enterococci in humans. In contrast, E. faecium and E. faecalis accounted for over 95% of the enterococci found in humans, but less than 5% of the enterococci in cattle. This suggests that cattle-associated enterococci may have a hard time making their way into humans.

The predominant Enterococcus species gradually shifted as sample collection sites moved from the cattle to the human environments. The farther the sampling moved away from the feedlot, the less common the cattle-associated E. hirae became. The closer the sampling moved towards the human environment, the more common the human-associated E. faecium and E. faecalis became. A big shift happened at processing and retail, where the human-adapted species suddenly increased to 75% of the enterococci collected. Human-adapted enterococci may move from people to beef in those environments, rather than from cattle to beef.

Antimicrobial resistance patterns also differed between environments. Resistance to macrolides (High Importance in Human Medicine) and tetracycline (Medium Importance) were commonly found in enterococci from feedlot samples, probably because of the extensive use of in-feed Tylan and chlortetracycline to control liver abscesses. Compared to samples from cattle-associated environments, enterococci from clinical patients and treated sewage treatment showed resistance to macrolides as well as antibiotics of Very High Importance in human medicine (e.g. antibiotics related to Baytril, Excede, Excenel and others). The antibiotic resistance patterns observed in cattle-associated environments reflected the types of antibiotics that are used in cattle, while the patterns observed in human-associated environments reflected the antibiotics used in human medicine.

Comparisons of stockpiled to composted manure found that composting was an effective way to dissipate antibiotic residues and degrade antibiotic resistance genes. A wide variety of antibiotic resistance genes was found in soil samples. This isn’t too surprising – many of the antibiotics used in human and veterinary medicine were originally discovered in soil bacteria. Bacteria naturally produce antibiotics to attack other bacteria and invade habitats that have better moisture, temperature or nutrient conditions, and they naturally develop antibiotic resistance to protect themselves from the antibiotics that other bacteria produce. As a result, microbiologically active soils appear to play an important role in degrading antibiotics and antibiotic resistance genes originating from manure.

What it means: Antibiotic resistant bacteria and genes are unlikely to flow from cattle to people through the environment when antibiotic use, manure and runoff are appropriately managed. Responsible antibiotic use slows the rate with which antibiotic resistance develops. Practices like manure composting help degrade antibiotic residues and antibiotic resistance genes. Adopting and documenting recommended antibiotic use and manure management practices will become increasingly important as public interest in and concern about beef production practices grows.

The Canadian Beef Cattle Check-Off has increased from $1 to $2.50 per head in most provinces, with approximately 75 cents allocated to the Beef Cattle Research Council to support research. Canada’s National Beef Strategy outlined why the Check-Off increase was needed, and how it would be invested. In Canada’s National Beef Strategy, the first target outcome under the Competitiveness pillar’s “Environment Sustainability” focus area is to continually improve environmental sustainability through validating the impacts of beef production. Answering these “what-if” research questions helps the industry prepare itself before these questions arise in the minds of consumers or regulators.

The Beef Research Cluster is funded by the Canadian Beef Cattle Check-Off and Agriculture and Agri-Food Canada with additional contributions from provincial beef industry groups and governments to advance research and technology transfer supporting the Canadian beef industry’s vision to be recognized as a preferred supplier of healthy, high quality beef, cattle and genetics.

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