Antibiotics are widely used in livestock, and when manure is re-purposed as fertilizer or bedding, traces of the drugs can leach into the environment, potentially contributing to the global antibiotic resistance crisis. Yet studies of antimicrobial residues in the environment have mainly focused on municipal wastewater effluents. Farmers typically use waste management systems to treat solid and liquid manure before re-use – reducing offensive odors and making it suitable for use as fertilizer. A team from University at Buffalo, New York, wanted to know how effective these farmyard systems are in removing antibiotic residues. Their ultimate goal? “We are looking for strategies to minimize the environmental dissemination of antimicrobial compounds and antimicrobial resistance genes to reduce the agricultural contribution to the emergence and spread of antimicrobial resistance,” explains Professor of Chemistry Diana Aga, lead researcher.
The team primarily used LC-MS/MS for the identification and quantification of antimicrobial residues in treated cattle manure. “The main analytical challenge we faced was developing sufficient extraction and clean-up procedures for achieving trace level detections, while preventing significant matrix suppression during analysis,” Aga says. They overcame this by performing extensive sample cleanup using solid phase extraction (SPE). The results indicate that standard waste management is not enough to remove antibiotic residues. The team found that tetracyclines were not completely removed during anaerobic digestion, and water recovered from the Livestock Water Recycling system (a reverse osmosis technique) was found to contain trace levels of ionophore antibiotics. Overall, solid waste contained higher levels of antibiotics than the liquid part of the manure.
“We were surprised to see that even high temperature conditions (such as pasteurization or composting) did not degrade the antimicrobials,” says Aga. “But it’s worth remembering that manure management strategies (including anaerobic digestion, composting, and so on) are designed to reduce odors, organic carbon and nitrogen levels – they are not specifically designed to remove antimicrobials.” The researchers would like to study transformation products of antimicrobials in manure using high-resolution mass spectrometry (HRMS), as well as digging into the effects that other contaminants have on antimicrobial resistance. The research can’t come soon enough, according to Aga. “We are running out of options for effective drugs to fight pathogenic antimicrobial-resistant bacteria – it is one of the grand challenges of our time.”
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