Worried about gross bacteria in your house? Get rid of pets - and men

More pets — and men — in your home means you are more likely to have some very yucky bacteria in your house, scientists say in a new study.

Bringing a pet into the house means importing the bacteria found in the animals' mouths and excrement, according to a new study of the microscopic organisms lurking in the dust of more than 1,100 U.S. houses. Human housemates have a similar effect: some bacteria that lurk on human skin and in human feces are more common in households with more men.

"Not everyone has the same bugs in their house," says study author Noah Fierer, a microbial ecologist at the University of Colorado, Boulder. "The best predictor of what bacteria you'll find in your house is whether you have dogs or cats" – and, to a small extent, the sex ratio of a home's human residents.

American houses with dogs were much more likely to teem with bacteria found in doggy breath or poop. Houses with a higher ratio of men to women recorded higher levels of Corynebacterium, a group of bacteria more abundant on the skin of men than the skin of women.

None of this should make the squeamish kick out the pets or the spouse, Fierer says. For starters, it's not clear how many of the microbes in the dust samples were alive. Even if they were, most won't bother us.

The public "shouldn't be worried," Fierer says. "We're constantly surrounded by microbes. … Some may be beneficial, most are probably innocuous."

To get a good look at our tiniest housemates, the researchers needed a wide sample of microscopic life forms – also known as microbes -- from across the country. So they recruited volunteers from across the United States to swab for dust both inside and outside their houses. The volunteers were instructed to swipe atop door frames, where grime lies undisturbed in all but the most fastidiously cleaned residences.

When the researchers analyzed the microbes' DNA, they learned that humans don't have much choice about the fungal portion of the home. For fungi, the key is location, location, location.

The fungi inside a house are largely the same as those floating around outdoors, and the fungi in a particular spot are determined by local factors such as average temperature, the researchers say in this week's Proceedings of the Royal Society B: Biological Sciences.

So the group of molds known as Aspergillus, which includes types that trigger allergies, was more common in the Northeast. The fungal group known as Alternaria, on the other hand, dominated in the Great Plains and Great Lakes states. The tight link between where a person lives and the fungi in his or her house mean that the only way to swap out fungal freeloaders is to move.

Researchers say the new study's enormous scope helps confirm findings from earlier analysis of small number of households.

The work "very nicely shows that there are these very broad patterns," says University of California, Berkeley microbial ecologist Rachel Adams, who was not affiliated with the new study. For fungi, "what's outdoors is determining what's indoors. But for bacteria … you have a little more control." She hopes that this study and others like it will help make people embrace the microbes that envelop them.

"We are swimming in a sea of micro-organisms," says microbial ecologist Jack Gilbert of the University of Chicago and Argonne National Laboratory. "We have to stop being squeamish about it and accept it as a reality."