A wastewater treatment plant's job description is pretty straightforward: Remove contaminants from sewage so it can be returned to the environment without harming people or wildlife.

But a new study suggests that the treatment process can have an unintended consequence of promoting the spread of extra-hardy bacteria.

Some bugs are resistant to antibiotics, so they dodge the medical bullets that wipe out others. The more drugs that are used, the more robust they become. Since bacteria reproduce quickly – one organism might turn into a billion overnight – and they share DNA with others, antibiotic-resistant genes spread like Darwinian wildfire when conditions are right.

And at sewage treatment plants, it seems, the conditions are right, said microbiologist Chuanwu Xi, whose University of Michigan lab conducted the study.

"Wastewater treatment plants are most effective at treating sewage when they have conditions that allow beneficial bacteria to thrive and improve the quality of the water," said Karen Kidd, a University of New Brunswick ecotoxicologist familiar with the study.

"However, this study indicates that these conditions can also favor the mutation of some and act as a source of antibiotic resistant bacteria to the environment."

"To me," she added, "that's sobering."

These "super" organisms in the treated sewage wind up in rivers and other waters, potentially infecting people with infections that are difficult to treat.

To determine if sewage-treatment plants might be a source of resistant bugs, Chuanwu and fellow researchers collected several species of the common bacteria Acinetobacter from a plant in Ann Arbor, Mich. that dumps its effluent into the Huron River.

They exposed the bacteria to various antibiotics and cocktails of drugs, and found a significant increase in the percentage of Acinetobacter that were resistant after each stage of treatment. And while the final treatment process killed all but a tiny fraction of the bugs before releasing the water to the environment, the proportion of resistant bacteria was much higher among those that made it back to the river than those collected upstream from the plant.

The bacteria were as much as 10 times more resistant to some antibiotics after secondary treatment at the Michigan plant. Also, in the river downstream of the plant, they were up to 2.7 times more resistant than bacteria upstream, according to the study.

Chuanwu said people and wildlife that swim in or drink from the Huron River downstream may be exposed to the more stalwart strains. However, the human health risk is not well understood.

Acinetobacter were chosen for their "remarkable ability" to develop resistance to antimicrobial agents, according to the Michigan study, which was published online in March in the journal Science of the Total Environment. The bacterium can cause pneumonia along with serious infections in wounds and in the bloodstream, according to the Centers for Disease Control and Prevention. Most infections affect people in hospitals, where common use of antibiotics promotes growth of resistant strains.

"We don't know whether other bacteria would respond to the treatment process in the same way the Acinetobacter did," Chuanwu said. "We have some unpublished data suggesting a similar trend of resistance increase among all bacterial populations."

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."

Cowles called the study's findings "very surprising" and said if they are accurate, "that's pretty significant," he said. "That particular facility puts out one of the cleanest effluents in the country. If they're really showing that, then that's a wakeup call."

"Wastewater operators are concerned" about antibiotic resistance, Cowles said, "but it's a matter of needing research."

It's also a matter of cost.

Treatment plants use chlorine or ultraviolet light, or both, to kill microorganisms before discharging effluent to the environment, and although "in general, it's relatively safe," neither method kills all bacteria, Cowles said. For the right price, though, plant operators could wipe them out through reverse osmosis or the use of activated carbon.

"Is it possible to sterilize it? Of course," he said. A project in Orange County, Calif., for example, uses reverse osmosis and other advanced technologies to render sewage discharge pure enough to recycle as drinking water.

"It's a matter of money," said Cowles. "But it's very unlikely that the American public would tolerate the cost of doing that."

It's also unclear whether the risk of letting a few bugs survive in effluent warrants the high cost of completely eradicating them, he added.

"The environment provides the opportunity for infection no matter where you are, upstream or downstream," he said.

Meanwhile, according to Steitz, there's an ongoing arms race between superbugs and the medical world.

"Evolution trumps intelligent design," he said. "Even though you get really smart drugs, they'll eventually get around it."

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.