Antibiotics promote resistance on experimental croplands — ScienceDaily

Canadian researchers have generated both novel and existing antibiotic resistance mechanisms on experimental farmland, by exposing the soil to specific antibiotics. The research is published in Applied and Environmental Microbiology, a journal of the American Society for Microbiology.

“Antimicrobial resistance is a global threat to human and animal health with bacteria now resistant to the last-resort antibiotics, including carbapenems and polymyxins,” said corresponding author Ed Topp, PhD, Principal Research Scientist at Agriculture and Agri-Food, Canada, London, Ontario, describing the motivation for the research.

Antibiotics have been getting into crop production systems via application of manures from livestock, which are given antibiotics to maintain health under crowded production conditions. Antibiotics also get into croplands when human waste is applied as fertilizer, and directly from livestock on pasture. (Certain different antibiotics used to be given to livestock to promote faster growth, but Topp said that in both Canada and the US, the regulations now prohibit that, and his experiments did not include those antibiotics.)

In the study, Topp and his collaborators, including first author Calvin Lau, PhD, a Visiting Fellow at Agriculture and Agri-Food, Canada, exposed farm plots to antibiotics, over periods of up to 16 years.

Topp’s research team ultimately sampled soil from the plots, extracting DNA, and cloning large fragments of that DNA into a strain of E. coli that is sensitive to most antibiotics. They then plated large numbers of E. coli onto a medium that contained one of several antibiotics, including from the classes of macrolides, ?-lactams, and sulfonamides.

“Any colonies that grew would presumptively contain a cloned fragment that conferred resistance,” said Topp. “We obtained a total of 36 antibiotic resistance genes from the antibiotic-resistant E. coli.”

The investigators compared the resistance genes with currently known resistance…

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