Flatmates and Couples Swap Gut Bacteria the Longer They Live Together

Who you live with may be quietly reshaping the microbial community inside your gut. A study from the University of Trento, analyzing microbial DNA from over 9,700 stool and saliva samples collected across more than 20 countries on five continents, finds that people who share a home progressively swap strains of gut and oral bacteria. The longer the cohabitation, the more similar the microbiomes become. That finding, while still emerging in its implications, raises some striking questions about how disease itself might spread.

Gut Microbiome Strain Sharing Between Cohabitants

First author Mireia Vallès-Colomer and group leader Nicola Segata found that couples shared microbial strains to a greater extent than parents and children did. This is notable because prior research using simpler tools had already suggested that household members share microbiota more than strangers do, but the Trento team deployed high-resolution metagenomic tools capable of distinguishing between individual strains of the same species. That level of precision, previously very difficult to achieve, let them trace exactly which microbial lineages were moving between people rather than just noting that two people harbored the same species in common.

The human gut microbiome is an extraordinarily complex ecosystem: over 1,000 species are known to inhabit it, making it far denser and more diverse than the oral microbiome. Documenting transmission at the strain level within that system is a significant methodological step forward.

How Close Social Contact Shapes Microbial Diversity

Couples and cohabiting partners showed more similar gut microbiota composition than individuals living apart, a pattern that echoes earlier work suggesting that cohabiting partners have higher gut microbiota diversity than unmarried, non-cohabiting individuals. The Trento study adds a geographic dimension: rural households showed more shared microbiomes than urban ones, suggesting that living environment and daily contact patterns both contribute.

The data also traced microbiome transmission across the lifespan. Babies share up to 50% of their bacterial strains with their mothers at birth, a figure the study describes as a "microbial starter kit." That proportion drops to 34% by age one and continues to decline as the child ages and encounters a broader social world. Notably, older adults between 50 and 85 still shared measurable microbiome overlap with their mothers, and children accumulated microbial similarities with their fathers over time as well.

How 9,700 Samples From 20 Countries Enabled the Study

The scale of the dataset is what gave the researchers enough statistical power to draw these conclusions. Stool and saliva samples came from individuals in more than 20 countries spread across five continents, and family members were included specifically to study household transmission. High-resolution metagenomics, which sequences the full DNA of a microbial community rather than targeting a single marker gene, allowed the team to distinguish strains that would look identical under older methods.

This matters because two people can both harbor a species like Bacteroides fragilis without sharing the same strain. Only by resolving strain-level identity can researchers determine whether a microbe genuinely moved from one person to another, rather than simply being a common environmental acquisition. Tracing specific strains through a population was previously extremely difficult, and this is one of the first studies to do it at this geographic breadth.

Open Questions About Microbiome Transmission and Disease Risk

The most provocative implication of the research concerns disease. Vallès-Colomer stated that "diseases that we nowadays consider non-communicable, such as cancer or heart disease, could have a transmissible microbial component." The logic runs roughly like this: if microbiome dysfunction is implicated in conditions like obesity, type 2 diabetes, and certain cancers, and if microbiomes are demonstrably shared between people in close contact, then at least some component of that disease risk might move between individuals.

This remains a hypothesis. Establishing whether a shared microbiome strain actually contributes to a health outcome, rather than merely correlating with shared lifestyle factors like diet and stress, is genuinely hard. Demonstrating microbiome causality requires controlled experimental models that are difficult to translate cleanly to human health outcomes, and the Trento study does not claim to have resolved that question. The researchers themselves acknowledge that much remains to be discovered.

A more diverse microbiome is generally hypothesized to be more resilient, which raises an interesting wrinkle: if cohabitation increases shared strains, does that also mean it increases diversity or decreases it? The study does not fully resolve this, but the association between rural cohabitation and greater microbiome sharing suggests that the built environment plays a role the field is only beginning to map.

What Comes Next in Microbiome Transmission Research

The study fills an important gap in understanding how people assemble and reformulate their microbiomes across a lifetime, from the bacterial handoff at birth to the gradual convergence that happens when adults share a home for years. The next logical steps involve following cohabiting individuals longitudinally to see whether microbiome convergence predicts shared health trajectories, and whether specific strains are consistently transmitted in directions that correlate with measurable outcomes.

For now, the finding is best read as a significant piece of epidemiological evidence rather than a clinical directive. The microbiome is shaped by who you live with; what that ultimately means for your health is a question that will take years of follow-up research to answer carefully.

Evidence rating

EMERGING — 80/100

Caveats:

• This is a large observational study — it shows association between cohabitation and microbiome similarity, not that sharing microbes causes measurable health changes.
• The article is a press summary of the underlying research; the full peer-reviewed paper should be consulted for complete methods, statistical controls, and species-level detail.
• Funding source for the University of Trento study is not disclosed in available reporting.
• Claims about non-communicable diseases (cancer, diabetes, obesity) having a transmissible microbial component are speculative and not established by this study.

Large-scale observational study analyzing metagenomic data from over 9,700 stool and saliva samples across more than 20 countries, using high-resolution strain-level tools to detect microbiome sharing among cohabitants. The cross-sectional and longitudinal design identifies associations between social proximity and microbiome similarity but cannot establish that microbial exchange itself causes downstream health effects. The strain-sharing findings in mother-infant pairs are consistent with prior literature, though the full peer-reviewed paper requires independent review for complete statistical and methodological detail.

References

1. (2026). Shared home, shared microbiome? Study says flatmates swap gut bacteria – Euronews. https://www.euronews.com/health/2026/06/17/shared-home-shared-microbiome-study-says-flatmates-swap-gut-bacteria
2. Se Jin Song, Christian Lauber, Elizabeth K Costello, et al. (2013). Cohabiting family members share microbiota with one another and with their dogs. eLife. https://doi.org/10.7554/elife.00458
3. Kimberly A. Dill-McFarland, Zheng-Zheng Tang, Julia H. Kemis, et al. (2019). Close social relationships correlate with human gut microbiota composition. Scientific Reports. https://doi.org/10.1038/s41598-018-37298-9
4. Wenqi Zheng, Fei Liu, Feng Du, et al. (2020). Characterization of a Sulfhydryl Oxidase From Plasmodium berghei as a Target for Blocking Parasite Transmission. Frontiers in Cellular and Infection Microbiology. https://doi.org/10.3389/fcimb.2020.00311