Introducing environmental microbiota to urban homes shifts the indoor microbiome

Introducing forest soil on an entryway doormat shifted the indoor microbiome of Finnish homes closer to bacterial profiles found outdoors, with less contribution from human-associated bacteria, a new study shows. In the future, such interventions rebalancing the home microbiome could be used for health promotion, especially in urban settings. The study was led by the Finnish Institute for Health and Welfare and the University of Eastern Finland.

“Applying forest soil onto a rug led to a clear rise in forest soil‑associated bacteria in the air. The effect was most pronounced at infant breathing height for the first two weeks after application, and the signal was also detectable in other areas of the home,” says lead author, Chief Researcher Martin Täubel.

Childhood home microbiome may influence long-term health

Because early childhood is spent largely indoors, the home microbiome is a major source of microbial exposures that activate children’s immunoregulatory pathways and may influence their long-term health. Reduced encounters with environmental microbes have been observed in urban homes and are linked to an increased risk of inflammatory diseases, including asthma and allergies. This has sparked interest in interventions to modify indoor microbial exposures toward health-promoting interactions.

In the present study, the researchers tested the feasibility of environmental microbiota transfer into urban Finnish homes by performing a simple forest soil-on-rug intervention. The study involved six homes, five of which took part in the intervention and one acted as the control. Forest soil was added to an indoor rug in the entryway three times at four‑week intervals. House dust was sampled before the soil was added and every two weeks at infant and adult breathing heights and from floors at different locations, and microbiota were analysed from the samples using DNA sequencing and qPCR. 

The intervention led to a significant increase in the relative abundance of forest soil bacteria  across the different measurement points within homes.  Further effects were seen at infant breathing height near the rug. There, both the Farm‑home Resembling Microbiota Index (FaRMI), previously associated with lower asthma risk, and bacterial diversity increased, and the proportion of human‑associated bacteria decreased.

“It was promising to see that microbial exposure signatures associated with a lower asthma risk may be increased in urban homes with such a simple, low‑cost intervention. The next step is to see whether this type of intervention translates into the health benefits we expect,” notes University Researcher Pirkka Kirjavainen, senior author of the paper.

The article was published in the journal Microbiome.