Past studies have examined the link between wastewater treatment and antibiotic resistance, but this is the first to look simultaneously at a plant and the water body that receives its effluent.
At sewage treatment plants, operators intentionally create conditions that promote growth of microorganisms in wastewater because they break down organic matter. In oxygenated waters with plenty to eat, those beneficial bacteria thrive and reproduce quickly. But so do their more harmful cousins. And because treatment plants create far higher densities of bacteria than exist in the environment, "they could very likely increase gene transfer among microorganisms," Chuanwu said.
Before the bacteria can build resistance, though, they have to be exposed to antibiotics. That's where the average citizen comes in. When people take antibiotics, a good deal of the drugs head to the treatment plant when toilets are flushed. The same is true when they dump unused medicine down drains.
"Most antibiotics are pretty stable, so up to 90 percent of them end up in the wastewater," said Chuanwu. "In order to deal with this problem, we need to think about how to wisely reduce the use of antibiotics."
The CDC lists antibiotic resistance among its top concerns, and warns that resistant strains can spread quickly through communities. Some bacteria, commonly called "superbugs," are so tough that no antibiotics exist that can cure infections.
The poster-child superbug is methicillin-resistant Staphylococcus aureus, or MRSA, a bacterium that in 2005 killed nearly 19,000 people in the United States alone. But more recently, the Acinetobacter bacteria have drawn attention and earned a bad reputation. A January report from the Infectious Disease Society of America said that a particular strain, Acinetobacter baumannii, along with other microbes called Pseudomonas aeruginosa and Klebsiella pneumoniae, could soon rival MRSA as a killer. It has also become notorious as a common infector and occasional killer of soldiers and veterans of the Iraq and Afghanistan wars.
Thomas Steitz, a biochemist at Yale University who researches new kinds of antibiotics, said it is unlikely that drugs in most sewage could be strong enough to cause resistance, but the University of Michigan's medical school in Ann Arbor could contribute already-resistant bugs that can share the resistant genes with other bacteria at the plant.
Resistant bacteria could also come from farm runoff, he said, since livestock at many large feedlots are regularly fed low doses of antibiotics.
Treatment plants do a fine job of removing most pollutants, said Jeff Cowles, an environmental engineer who used to oversee treatment plants for the Michigan Department of Environmental Quality, but they're ill-equipped to get rid of so-called "microconstituents" like pharmaceuticals, pesticides and nanoparticles.
"And we just don't know what's happening to them once they enter the system," Cowles said. "It's reminiscent of the 1950s when DDT was going into the environment. We just assumed that it was going away, but it wasn't going away."