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When wastewater treatment plants discharge treated water into rivers and lakes, they can also pass along antibiotic-resistant bacteria and their resistance genes, according to a new study (Environ. Sci. Technol., DOI: 10.1021/es202775r). If other bacteria in the environment snag these genes, municipal wastewater could contribute to the growth of antibiotic resistance worldwide, the researchers say.

When examining sources of antibiotic resistance, scientists have previously focused on agricultural waste, because farmers feed the drugs to animals to promote growth. But antibiotic resistance can also develop in the guts of people taking the drugs. Those resistant microbes get flushed into sewage and end up at wastewater treatment plants. Most treatment systems release some bacteria into the environment.

Timothy M. LaPara, an associate professor of environmental engineering at the University of Minnesota, Twin Cities, and his colleagues wanted to determine if a wastewater treatment plant in Duluth, Minn., releases bacteria resistant to tetracyclines, a group of common antibiotics. Every day, the plant discharges 40 million gal of treated water into the St. Louis River. The researchers picked the plant because it uses a treatment step that can remove large amounts of bacteria from wastewater and that many other plants don’t use. The plant also discharges into water that is free from other major sources of bacteria.