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.
The researchers first collected surface water samples upstream and downstream of the treatment plant. Through quantitative polymerase chain reaction tests on bacteria in the samples, they found that the water farthest from the plant--upstream on the St. Louis River and the areas of Lake Superior where the river empties--had low bacterial loads and nondetectable levels of antibiotic resistance genes.
But when the team examined the plant’s treated wastewater, they measured high levels of three tetracycline resistance genes, with one gene at a concentration of 18,000 gene copies per mL of water. They could still detect the genes a mile from the plant, albeit at concentrations 95% lower than those in the treated water.
LaPara says that this plant’s release of resistant bacteria despite its advanced treatment system suggests that wastewater treatment plants, in general, could be significant sources of antibiotic resistance genes.
The findings also worry Alice C. Layton, a professor of biochemistry at the University of Tennessee, Knoxville. She points out that unlike most antibiotic-resistant bacteria from agricultural waste, bacteria from municipal sewage already have the ability to colonize the human gut and infect people. However, she says, the relative risks of antibiotic resistance genes released from municipal water treatment and those released from agriculture are unclear.
Layton also cautions that, while scientists suspect that the bacteria found in municipal wastewater can spread resistance genes to other types of bacteria in the environment, there haven’t been studies confirming that those transfers happen.