Spreading antibiotic resistance in wastewater treatment

Michael Torrice in Chemical & Engineering News: When people pop antibiotics to treat infections, the drugs often end up excreted into sewage. As scientists continue to find these antibiotics in the wastewater coming from homes and hospitals, they worry that the drugs' presence is fueling the spread of antibiotic resistance. At the American Chemical Society meeting in Anaheim, Calif., researchers reported that wastewater contains other chemicals that might also promote antibiotic resistance: heavy metals.

Environmental scientists have previously observed a connection between metals and antibiotic resistance in metal-contaminated soils and freshwater sediments. The bacteria living in these environments had significantly higher levels of resistance than bacteria from noncontaminated soils.

Edward F. Peltier of the University of Kansas, Lawrence; David Graham of Newcastle University, in England; and their colleagues wondered if the phenomenon also occurred in wastewater treatment plants. These plants are a unique environment where, along with antibiotics, metals such as zinc and copper are common. Bacteria also play a key role in the treatment process. After removing solids from wastewater, treatment plants mix it with a sludge containing an array of bacteria that chew up dissolved organic compounds.

…They found that copper alone, in the absence of antibiotics, could promote resistance to the antibiotic ciprofloxacin, with resistance levels jumping from a baseline level of 7% of the reactor population after the first phase to 11% after the second phase. Zinc alone didn't have an effect, but in the presence of certain antibiotics it did enhance resistance levels. In reactors receiving zinc and tetracycline, 63% of the bacteria were resistant to the antibiotic. Meanwhile, resistance levels in reactors that received tetracycline and no metal were only 44%.

If metals do help spread antibiotic resistance in wastewater treatment plants, then they could be a more long-lasting source of resistance than are antibiotics themselves, Peltier says. "Antibiotics can degrade, metals can't," he says…

A wastewater treatment plant, shot by Kristian Bjornard, Wikimedia Commons via Flickr, under the Creative Commons Attribution-Share Alike 2.0 Generic license