# Environmental microbiota transfer from forest soil into urban homes: a proof-of-principle study

## TL;DR

This study shows that adding forest soil microbes to home rugs can change indoor bacteria, potentially improving health in urban homes.

## Contribution

Demonstrates feasibility of transferring environmental soil microbiota into urban homes to influence indoor microbial composition.

## Key findings

- Forest soil bacteria increased in house dust after seeding rugs, especially near the source.
- Effects were strongest in homes with low microbial influx like no pets and mechanical ventilation.
- Bacterial diversity and asthma-protective microbes increased in dust near the rugs.

## Abstract

Urban lifestyles are characterized by reduced encounters of environmental microbe stimuli that activate immunoregulatory pathways. This has been linked to an increased risk of inflammatory diseases, asthma, and allergies. A potential preventative solution is to modify indoor microbial exposures toward health-promoting interactions. Here, we test the feasibility of environmental microbiota transfers into urban homes and quantify the spatiotemporal impact on the built environment microbiota.

House dust microbiota of six Finnish homes was monitored over a 20-week period by collecting settled dust from infant (IBZs) and adult breathing zones (ABZs) and floor dust from different home locations. Microbiota in dust samples was characterized using qPCR and amplicon sequencing of the bacterial and archaeal 16S rRNA gene and fungal ITS1 region. Microbiota transfers were performed with repeated seeding of forest soil onto rugs placed in the home entryway.

We observed significant, post-intervention increases in the relative abundances of forest soil bacteria in house dust. The magnitude of effect was influenced by building characteristics, spatiotemporal dynamics, and occupant dynamics and was greatest in a home with comparably little additional microbial influx—a home with no pets, low occupancy, and mechanical ventilation. The most pronounced effect was observed in settled dust close to the soil-seeded rugs at IBZ, within the first 2 weeks after each seeding event, though the soil-associated bacterial signal also extended spatially into the living areas of the homes. Increases in bacterial diversity and an asthma protective microbiota index, as well as decreases in the proportion of human-sourced bacteria, were also observed, but only in airborne dust close to the soil-seeded rug. Effects on fungal microbiota or on the bacterial and fungal loads in house dust were inconsistent.

We demonstrate that a simple soil-to-rug intervention can modify the bacterial microbiota in airborne particulate matter in residential homes. The introduction of specific environmental soil microbes was most pronounced closest to the source, which is relevant when targeting infant inhalation exposure. While this approach is promising, specifically in highly urbanized settings, dosage and composition of environmental microbiota additions to reach health benefits require further study.

Video Abstract

Video Abstract

The online version contains supplementary material available at 10.1186/s40168-026-02352-6.