Dust from 1100 homes was surveyed to learn more about microbes that might exacerbate breathing problems and allergies ... or even have health benefits

If the steady build-up of grime, stains and crumbs on the floor is not enough to spur you to a home cleaning spree, then research on the bugs that lurk in house dust might just do the job.



Tests on dust gathered from the tops of door frames in more than 1100 US homes revealed rich communities of fungi and bacteria that varied with the sexes of the home’s occupants, and whether cats or dogs lived with them.



Scientists embarked on the study to learn more about the microbes in house dust that might exacerbate breathing problems, trigger allergies, or even have health benefits.

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“Every time we breath in we are inhaling hundreds or thousands of species and we mostly don’t understand which ones are bad for us, and which are beneficial,” said Robert Dunn, an ecologist who took part in the study at North Carolina State University. “We don’t even know what determines why you breathe in different ones in New Jersey and East London.”

The researchers asked volunteers on a citizen science project called The Wild Life of Our Homes to collect dust from the tops of door frames on the inside and outside of homes across the US. They picked the tops of door frames because they are rarely cleaned, and serve as “passive collectors of settled dust in our homes”, according to a report in Proceedings of the Royal Society B.



House dust is an unappealing muesli of skin flakes, food crumbs, soil particles, fabric fibres, animal fur, decomposing insects and their faeces. Living in and on the mixture are microbes that vary with climate, geography, local plant and animal populations, and the house occupants themselves.

The scientists counted more than 70,000 types of fungi and over 125,000 kinds of bacteria in the course of their investigation, and discovered that they could predict whether households had cats or dogs from the microbes in their dust alone. The same dust microbes hinted at whether homes had more males or females, as men and women can carry different types of bacteria, and shed them at different rates.

Two sorts of skin bacteria, Corynebacterium and Dermabacter, were more plentiful in homes with more males. Levels of Roseburia, a kind of bacteria found in human faeces, were apparently higher too. That might owe something to differences in hygiene practices, the scientists offer.

In homes with more women, a type of bacteria called Lactobacillus was often more abundant. The scientists attribute the source of these particular organisms to the “vaginal microbiome”, the ecosystem of microbes that set up home there.



Fungi and bacteria are more diverse inside homes than outside, because the outdoor species are carried in on shoes and clothing, or blow in on the wind, where they mingle with microbes that are typically found indoors.



Indoor dust was more likely to host moulds and fungi that attack wood, but they also had higher levels of fungi found on human skin, such as Candida and Trichosporon. The types of fungi were tied to geographical locations, but not to who lived in the homes. Residences in the Eastern US had relatively high levels of Agaricomycetes, a class of fungi that includes most edible mushrooms. In Western US homes, fungi that grow on decaying leaves and cause common plant diseases were more common.



While the fungi found in house dust were closely linked to local environment, it had little impact on the bacterial populations. Inside people’s homes, the bacteria in dust depended on who, or what animals, lived there. DNA analysis found traces of bacteria that lurk on human skin and in the faeces of household insects.



Pets had a huge effect on household bacteria. Owning a dog seemed to raise the levels of 56 different types of bacteria, including Porphyromonas and Moraxella. Cats, meanwhile, raised the levels of 24 types, including Prevotella and Sporosarcina, according to the study. Most of the bacteria linked to cats and dogs are found in their saliva or faeces.



“The pet effect on bacterial communities is in part caused by these pets directly shedding these bacterial taxa from their bodies into our home environment,” the scientists write. From the bugs found in house dust alone, they could predict whether a dog lived at the home with 92% accuracy, and a cat with 83% accuracy.



The researchers believe the work is a first step towards understanding the factors that lead to different types of microbes living in homes, and in turn, how they affect people’s health. Ultimately, Dunn said, it might be possible to fill homes with microbes that improve human health. Until then, the options are limited.

“If you want to change the types of fungi you are exposed to in your home, then it is best to move to a different home, preferably one far away,” the authors write. “If you want to change your bacterial exposures, then you just have to change who you live with in your home.”

