By Brian Owens, Inside Science

(Inside Science) -- Sampling the waste in a city's sewage system can be a good way to study the microbes that live in the population's guts – and could even offer a way to monitor public health issues such as obesity, according to new research.

The community of microbes that live in a person's gut, known as the microbiome, is intricately tied to that person's health. The microbiome can influence, and be influenced by, a range of characteristics such as weight, disease, diet, exercise, mood and much more. But it can be difficult to draw large-scale conclusions about what constitutes a "healthy gut" because of the financial and privacy implications of sampling large enough numbers of people.

So a team of researchers led by Sandra McLellan at the University of Wisconsin-Milwaukee and Mitchell Sogin at the Marine Biological Laboratory in Woods Hole, Massachusetts, set out to test whether they would be able to spot human microbes lurking in the soupy mix of municipal sewage systems, and thus sample entire cities at once.

"With a random sampling of a million people, we can start to look for trends in how human populations might be different in their microbiomes, according to their demographics," said McLellan.

The team gathered samples over the course of a year from sewage treatment plants in 71 different cities in 31 states, chosen for their geographic spread and range of obesity rates. The leanest city sampled was Steamboat Springs, Colorado, with an obesity rate of 13.5 percent, while the heaviest was St. Joseph, Missouri, with a rate of 37.4 percent.

Bacteria from human waste make up just a small proportion of the bacteria found in sewage. But using DNA sequencing, the researchers could identify genetic markers for most of the types of bacteria found in human feces in their samples. By comparing samples from the different cities, they identified a core group of bacterial families that the U.S. population seems to have in common.

After they compared how the proportions of human bacteria varied between cities, McLellan said they were able to see patterns that tracked characteristics like obesity with surprising accuracy. Using just the bacteria's genetic traces they were able to predict whether a city was lean or obese with almost 90 percent accuracy.

"If we had a city with a higher percentage of obese people, we would see a higher percentage of the kinds of bacteria associated with obesity," she said.

Randy Seeley, an obesity researcher at the University of Michigan in Ann Arbor, said it had never occurred to him that it would be possible to extract such information from the mishmash of organisms in sewage.

"Had they shown up in my office with this idea I would have said 'you're nuts, there's no way you can pull that off,'" he said. "The fact that they are capable of doing it just shows you the power of the big-data approach."

McLellan foresees two possible uses for this proof-of-concept study in the future. One is to harness the huge numbers of people being sampled in diverse cities to help determine which specific types of bacteria might be important for different aspects of human health. Currently, they have only been able to narrow it down to rather coarse groupings of bacterial families.

That will require samples from more cities over a longer timeframe, she said. "With enough cities, we should be able to start associating demographic traits with particular groups of bacteria."

But once reliable markers of certain traits are identified, McLellan also thinks the techniques developed by her team could be used to monitor the health of a city's population by sampling its sewage.

"Once you identify bacteria that are associated with healthy people, you can see where people are healthy or unhealthy based on their microbiome," she said. "So I think it does have some long-term surveillance applications."

Seeley isn't convinced that sewage sampling will be an effective surrogate for existing large-scale health surveys such as the Centers for Disease Control's National Health and Nutrition Examination Survey.

"I'm somewhat skeptical that sewage sampling would be better or cheaper than getting individual results from the survey," he said.

But he does see a use for it in assessing whether large-scale health interventions – programs on a similar scale to putting fluoride in drinking water – are having any effect on the target population.

"You might be able to find biomarkers of community change that would help you assess how well that intervention was going in community A versus community B," he said.

The researchers published their results in a recent issue of the journal mBio.

Brian Owens is a freelance science journalist in St. Stephen, New Brunswick, Canada. Reprinted with permission from Inside Science, an editorially independent news product of the American Institute of Physics, a nonprofit organization dedicated to advancing, promoting and serving the physical sciences.