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updated 8/19/2011 4:32:14 PM ET 2011-08-19T20:32:14

There’s a new reason to crack down harder on dog owners who don’t clean up after Fido. Samples in two cities found that in winter the most common bacteria in the air is from feces — probably that of dogs. Researchers want to extend their air sampling to cities across the country to see how widespread the bacteria might be.

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"A significant percentage, anywhere from 10 to over 50 percent of the bacteria, seem to be derived from feces," Noah Fierer, an assistant professor of ecology at the University of Colorado at Boulder, told msnbc.com.

Fierer and colleagues looked at air samples taken in winter from four cities in the Midwest — Chicago, Cleveland, Detroit and Mayville, Wis.

Chicago and Mayville did not show elevated levels, but Cleveland and Detroit did.

"To our surprise the airborne bacterial communities of Detroit and Cleveland most closely resembled those communities found in dog poop," lead author Robert Bowers, a University of Colorado graduate student, said in a statement.

"As best as we can tell, dog feces are the only explanation for these results," added Fierer. "But we do need to do more research."

"We only had access to those samples from the Midwest," Fierer said, "but it is entirely possible that these cities are not at all unique — we just don't know. We are currently pushing for a continental-scale atlas of outdoor air bacteria in order to address some of these basic questions.

"Long story short," Fierer said, "we have no idea if the patterns observed in the studied cities are unique to those cities or more widespread."

The researchers focused on air samples in winter, when snow and fallen leaves tamp down other sources of bacteria. "The soil and leaf surfaces become far less important and the 'relative' abundance of fecal-derived bacteria seems to become far higher," said Fierer.

That humans are exposed to bacteria in the air is not new.

"We breathe in bacteria every minute we are outside, and some of these bugs may have potential health implications," Fierer said. "We need much better information on what sources of bacteria we are breathing in every time we go outside." 

So what cities might be sampled next? "We are still looking for funding to support this work," Fierer said, "so no details yet."

The team reported its findings in a study published in Applied and Environmental Microbiology.

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