Scientists break new ground on manure recycling and antibiotic resistance link

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Spreading manure on land may be contributing to increased antibiotic resistance in soil.
Spreading manure on land may be contributing to increased antibiotic resistance in soil. © The Donkey Sanctuary

Composting manure may be one of the keys to fighting antibiotic resistance, as scientists look at the overuse of veterinary antibiotics in animal production and the subsequent land applications of manure.

While the problem stems from mainly from the output of food animals, the subsequent land applications of manure contribute to increased antibiotic resistance in soil.

Sampling manure compost for antibiotic resistance gene measurement
Sampling manure compost for antibiotic resistance gene measurement.

A new review published in the European Journal of Soil Science examines the results of recent studies on veterinary antibiotic use, the concentrations of antibiotics, and the abundance and diversity of antibiotic resistance genes in animal manure and in soil that receives manure or manure composts.

The review also discusses the need for more stringent regulations on the use of veterinary antibiotics and future research directions on the mechanisms of antibiotic resistance and resistance management.

“Recycling of animal manures to soil is good for soil quality, but the spread of antibiotic resistance needs to be tackled urgently,” said co-author Dr. Fang-Jie Zhao, of Nanjing Agricultural University, in China.

Small proportions only of antibiotics are absorbed or metabolized by animals, with most of the antibiotics being excreted in faeces. In China alone, for example, the researchers noted that annual antibiotic residues in manures can reach 29,000 to 87,000 tons. In 2013, the total use of 36 antibiotics within the five major antibiotic categories (fluoroquinolones, beta-lactams, macrolides, sulphonamides and tetracyclines) was 92,700 tons, 84.3% of which was veterinary antibiotics. The excreted amount of the 36 antibiotics was estimated to be 53,800 tons, of which 84.0% was produced by animals.

Composting, especially thermophilic composting, could degrade between 50 and 99% of some antibiotics, such as tetracyclines, in the manure. But, in some cases, antibiotics such as sulphamethazine, ofloxacin and ciprofloxacin can be resistant to the composting treatment and remain at large concentrations in the composted products.

The antibiotic resistance gene (ARG) abundance in manures and manure-based composts. CM, cattle manure; PM, poultry manure; CC and PC, thermophilic compost product from cattle manure and poultry manure, respectively; MLSB, macrolide-lincosamide-streptogramin B resistance genes.
The antibiotic resistance gene (ARG) abundance in manures and manure-based composts. CM, cattle manure; PM, poultry manure; CC and PC, thermophilic compost product from cattle manure and poultry manure, respectively; MLSB, macrolide-lincosamide-streptogramin B resistance genes.

Meanwhile, researchers are busy devising strategies to address the threat of antibiotic resistance to human health – and Canada’s Université de Montréal is at the forefront of the fight.

One of the ways antibiotic resistance genes spread in hospitals and in the environment is that the genes are coded on plasmids that transfer between bacteria. A plasmid is a DNA fragment found in bacteria or yeasts. It carries genes useful for bacteria, especially when these genes encode proteins that can make bacteria resistant to antibiotics. A team of scientists at UdeM’s Department of Biochemistry and Molecular Medicine has come up with a novel approach to block the transfer of resistance genes. The study by Bastien Casu, Tarun Arya, Benoit Bessette and Christian Baron was published in early November in Scientific Reports.

The researchers screened a library of small chemical molecules for those that bind to the TraE protein, an essential component of the plasmid transfer machinery. Analysis by X-ray crystallography revealed the exact binding site of these molecules on TraE. Having precise information on the binding site enabled the researchers to design more potent binding molecules that, in the end, reduced the transfer of antibiotic-resistant, gene-carrying plasmids.

Building on their encouraging new data, Baron and his colleagues are now working with the medicinal chemists at UdeM’s IRIC (Institut de recherche en immunologie et cancérologie) to develop the new molecules into powerful inhibitors of antibiotic resistance gene transfer. Such molecules could one day be applied in clinics in hospitals that are hotbeds of resistance, Baron hopes.

Ultimately, reducing the transfer of antibiotic-resistance plasmids could help preserve the potency of antibiotics, contributing to an overall strategy to help improve human health, he said.

“They say that by 2050, 50 million people will die from antibiotic resistant infections,” said the Toronto-born, German-raised researcher. “The day when we can’t treat infections with antibiotics is coming. Nevertheless, people should have hope. Science will bring new ideas and new solutions to this problem. There’s a big mobilization now going on in the world on this issue. I wouldn’t say I feel safe, but it’s clear we’re making progress.”

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