SICKENING SLUDGE? Some bacteria is a good thing for sewage treatment, but good bacteria turn bad when they become resistant to the melange of drugs that end up in the water system.
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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."