UWM professor Ryan Newton looks through sewer pipe segment.

The alarming (in)side of sewage pipes

School of Freshwater Sciences researchers Ryan Newton and Sandra McLellan were developing new methods for tracking fecal pollution when they made a surprising discovery: Bacteria from human waste accounted for only a small portion of the microbes found in sewage.

What they found in abundance were unusual bacterial communities. These consisted of a particular combination of microbes that don’t exist anywhere else in nature. But they appear consistently in city sewerage systems regardless of where they’ve been sampled around the world.

Ryan Newton and Sandra McLellan discovered a combination of microbes that consistently appear in city sewerage systems but don’t exist anywhere else in nature.

“The sewerage system has only been around for the past 100 to 120 years,” says Newton, an assistant professor of freshwater sciences. “So members of this microbiome are like squirrels and raccoons that have adapted to live in cities.”

Wastewater infrastructure is designed, in part, to remove microorganisms, Newton says, but these communities have evolved to linger and thrive in their secluded environment. Although scientists don’t yet know the exact residents of the microbiome, he says it’s urgent that they learn more.

Results from a previous study conducted by Newton suggested sewerage pipes are a hot spot for antibiotic-resistant bacteria to transfer that trait to other infection-causing microbes. The pipe microbiome could be the reason.

The prevalent use of prescription drugs means more and more bacteria are becoming resistant to antibiotics of last resort. When those microbes pass through our bodies, they usually end up in the sewerage system, where resistant and unaffected pathogens interact.

Newton and McLellan, a professor of freshwater sciences, have sequenced the DNA of microbes in sewage samples from 71 cities. They’ve noticed that the number of antibiotic-resistant genes increases a hundredfold once they enter sewerage infrastructure. For every antibiotic-resistant gene that enters the pipes, there are 100 upon arrival at the treatment plant.

And the treatment plant doesn’t solve the problem. Roughly 100,000 microbes per liter of wastewater have resistant genes after treatment, Newton says.

To quantify the impact these microbes have on human and ecosystem health, researchers must first learn which bacteria are of most concern and then determine what factors encourage the spread of resistance.

“People shed antibiotic-resistant bacteria wherever they go,” Newton says. “Sewage is another way that resistance can be transmitted from place to place.”