The dust that gathers in most households would seem to be little more than a nuisance for those charged with cleaning it off windowsills and coffee tables.
But for scientists, it's a trove of invisible bacteria and fungi providing a unique window into our homes.
From these microscopic critters, researchers from the University of Colorado Boulder and North Carolina State University have discovered that it is possible predict the geographic region of a given home, the gender ratio of the occupants and, less surprisingly, the presence of a pet.
"Every day, we're surrounded by a vast array of organisms in our homes, most of which we can't see," said Noah Fierer, an associate professor in the Department of Ecology and Evolutionary Biology at CU-Boulder and a co-author of the study, which was published this week in the journal Proceedings of the Royal Society B. "We live in a microbial zoo, and this study was an attempt to catalog that diversity."
The study examined roughly 1,200 homes across the continental United States. Volunteers, recruited online, participated in a project called Wild Life of Our Homes. They were asked to swab the inside and outside of their front doors and send those samples in for analysis.
"When we started this project, we wanted to see what invisible lifeforms were sharing our homes," said NC State biology professor and study co-author Rob Dunn.
"Which species of fungi and bacteria live with us? And what's responsible for determining which species live in which homes? No one really knew," he said. "But we also wanted to devise a study that stirred the public imagination, and made the public a meaningful part of the scientific process."
On average, each home contained more than 5,000 different species of bacteria and around 2,000 species of fungi. In their study, the researchers found genetic traces of more than 72,000 taxa of fungi and more than 125,000 taxa of bacteria.
"We found tens of thousands of bacteria that no one knows anything about - they don't even have names," Dunn said.
The bacteria didn't vary much from from one part of the country to another. A rural home, for example, was likely to have similar populations of bacteria to an urban home thousands of miles away.
But there were factors that appeared to influence bacterial biodiversity. Homes that had a cat appeared to favor specific bacteria. Homes with dogs appeared to favor other bacteria. And the types of bacteria found in a home were also influenced by the ratio of men to women in a home.
"We can tell if there are more men than women in a home, for example, because those homes have more armpit bacteria," Dunn says. "Seriously."
While bacterial communities provide clues about the identity of its residents, the researchers said fungal communities are more predictive of a home's location.
"Geography is the best predictor of fungi in your home," said Fierer, who is also a research fellow at the Cooperative Institute for Research in Environmental Sciences at CU-Boulder. "The reason is that most fungi blow in from outdoors via soil and leaves."
And once you have a microbe profile, it can be pretty hard to change it.
"One of the key takeaways is that if you want to change what you breathe inside your house, you would either have to move very far away or change the people and the pets you live with," said Albert Barbarán, a graduate researcher in CU-Boulder's Department of Ecology and Evolutionary Biology and lead author of the study.
The researchers called this a "first step" to profiling the invisible world in our homes. Among other things, they said findings could have implications for future forensics work or allergen research.
"We're just starting to look at what lives in our homes," Dunn says. "These findings are not an exhaustive answer, they're a first step - and the study highlights just how much we don't know."