Scientists say the type of bacteria and fungi vary by place of residence and their residents, but point out that most of them are not harmful to health.
A new scientific study suggests that we may be sharing our homes with about 9,000 different species of microbes.
Researchers at the University of Colorado, United States, analyzed dust collected from 1,200 US households and found that this is the average of the types of bacteria and fungi found in the dust of an ordinary home.
Published in "Proceedings of the Royal Society B.", the study is part of the "The Wildlife of Our Homes" project, where volunteers sent samples of dust accumulated on the doorjamb, places that are often overlooked during cleaning, according to scientists.
Image shows dust seen under the microscope
"We have long known that microbes inhabit our homes. What we are doing now is apply good old science to see how they vary according to where they are found," says Noah Fierer, professor of ecology and evolutionary biology.
Researchers have come to the conclusion that a common house usually has more than 2,000 different types of fungus and that its type varies according to the location of the residence.
"Most of the fungi found in houses apparently come from outside," says Fierer. "They come in through clothes or open windows and doors. So the best way to predict what kind of fungus lives in a house is to see where it is."
Scientists have even discovered an average of 7,000 different bacteria per household. Some of these are related to human skin, but faecal species have been found.
In this case, the variety depends not on the location of the house, but on who inhabits it.
"We found different bacteria in houses where women live than in those inhabited only by men," says Fierer. "There are types of bacteria more commonly associated with the female body than the male, and we could see it in the dust."
The presence of pets also influenced the variety of microbes.
"Having a cat or dog significantly impacts the bacteria found in a home," Fierer explains. "It was surprising for us to find such an influence - greater than any other factor."
Although the study was conducted in the United States, the scientist says the results are relevant to homes in other parts of the world.
Now researchers want to find out how these microorganisms can affect human health. While some are linked to disease and allergies, most are probably harmless, scientists say, and may even be beneficial.
"People don't have to worry about microbes in their homes. They're around us, in our skins, in our homes - and most of them don't hurt," says Fierer.