University of Colorado researchers sampling the winter air in four Midwestern cities say the bacterial colonies found in Cleveland and Detroit most closely resemble that found in dog poop.

In their study, published July 29 in “Applied and Environmental Microbiology,” the researchers compared the DNA of collected bacteria to a database of bacteria from known sources such as leaf surfaces, soil, and human, cow and dog feces.

“We found unexpectedly high bacterial diversity in all of our samples, but to our surprise the airborne bacterial communities of Detroit and Cleveland most closely resembled those communities found in dog poop,” said lead author Robert Bowers, a graduate student in CU’s ecology and evolutionary biology department and the Cooperative Institute for Research in Environmental Sciences, or CIRES, based at CU. “This suggests that dog poop may be a potential source of bacteria to the atmosphere at these locations.”

The researchers were trying to better understand exactly what microbes circulate in urban environments, CU said in a news release. They sampled air in summer and winter in Chicago, Cleveland and Detroit, and the tiny town of Mayville, Wisc.

The team also used nearly 100 air samples collected as part of a previous study Colorado State University researchers assessing the impact of residential wood burning and prescribed fires on airborne fine particle concentrations in the American Midwest.

“What we’ve been looking at are the numbers and the types of bacteria in the atmosphere,” said Noah Fierer, an assistant professor in CU’s ecology and evolutionary biology department and a CIRES fellow. “We breathe in bacteria every minute we are outside, and some of these bugs may have potential health implications.”

Fierer said scientists already knew that bacteria exist in the atmosphere and that these bacteria can have detrimental effects on human health, triggering allergic asthma and seasonal allergies.

In recent years, however, researchers have realized that there is an incredible diversity of bacteria in the air. “We are just starting to realize this uncharted microbial diversity in the air,” Fierer said.

In the summer, airborne bacteria come from many sources including soil, dust, leafsurfaces, lakes and oceans, Bowers said. But in the winter, as leaves drop and snow covers the ground, the influence that these environments have as sources also goes down. It is during this season that the airborne communities appeared to be more influenced by dog feces than the other sources tested in the experiment, he said.

The team plans to investigate the bacterial communities in other cities and to build a continental-scale atlas of airborne bacterial communities, Fierer said. “We don’t know if the patterns we observed in those sites are unique to those cities,” he said. “Does San Francisco have the same bacteria as New York? Nobody knows as yet.”

Fierer believes it is important to pin down the types of bacteria in the air, how these bacteria vary by location and season, and where they are coming from. With this information, scientists can then investigate the possible impacts on human health, he said.

“We need much better information on what sources of bacteria we are breathing in every time we go outside,” Fierer said.

The study’s co-authors included: Rob Knight, an associate professor in CU-Boulder’s chemistry and biochemistry department; Amy Sullivan and Jeff Collett Jr. of Colorado State University in Fort Collins; and Elizabeth Costello of the Stanford University School of Medicine.