The air of two Midwestern US cities contains significant amounts of bacteria from feces, particularly dog poop, according to a new study led by the University of Colorado Boulder, published recently in the journal Applied and Environmental Microbiology. The researchers tested the air of four locations in the Great Lakes region, and found that, in addition to the more predictable organic sources (such as leaves and soil), fecal material, most likely from dogs, often represents an unexpected source of atmospheric bacteria in urbanized areas during the winter.

Lead author Robert Bowers, a graduate student at the University of Colorado Boulder and Cooperative Institute for Research in Environmental Sciences (CIRES), told the press on Thursday:

“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.”

The atmosphere around and above us is full of material of organic origin, a significant amount being bacteria, including those that cause disease in plants and animals. Some of them also cause or trigger human diseases, including asthma and seasonal allergies.

Yet, despite their importance, we don’t know a lot about them, and it is only in the last few years that scientists have begun to realize the diversity of bacteria in the air:

“There is a real knowledge gap,” said co-author Noah Fierer, an assistant professor in CU-Boulder’s ecology and evolutionary biology department and a CIRES fellow. “We are just starting to realize this uncharted microbial diversity in the air — a place where you wouldn’t exactly expect microbes to be living”.

“We breathe in bacteria every minute we are outside, and some of these bugs may have potential health implications,” he added.

For the study, the team analyzed the numbers and types of bacteria in nearly 100 local air samples, taken in summer and winter, at four places in the Great Lakes region of the US: three large cities of more than 2 million inhabitants (Chicago Cleveland and Detroit), and one town with less than 6,000 people (Mayville in Wisconsin).

They looked for bacterial DNA in the air samples and compared it to a database of known DNA signatures of bacteria from leaf surfaces, soil, and human, cow and dog feces.

They found a surprising diversity in the atmospheric bacterial communities. Also, in the winter samples from two of the four locations, Detroit and Cleveland, the bacterial DNA signatures most closely resembled that of dog feces.

“As best as we can tell, dog feces are the only explanation for these results,” said Fierer, adding that they need to do more research before they can be sure.

Bowers suggested that because the snow covers the ground in the winter, the proportion of airborne bacteria from leaf surfaces, soil and dust goes down.

Fierer said they don’t know if the patterns they found in these four cities are unique:

“Does San Francisco have the same bacteria as New York? Nobody knows as yet,” said Fierer.

“We need much better information on what sources of bacteria we are breathing in every time we go outside,” so that scientists can then investigate their impact on human health, he added.

The team now wants to investigate other cities and create a continental “atlas” of airborne bacterial communities.

Funds from CIRES (a joint institute of CU-Boulder and the National Oceanic and Atmospheric Administration), the US Environmental Protection Agency, the National Science Foundation, the Howard Hughes Medical Institute and the National Institutes of Health helped pay for the research, with additional support from the Lake Michigan Air Directors Consortium for air sample collection.

Written by Catharine Paddock PhD