Summary: Researchers mapped the precise environmental and biological dynamics that govern strain-level transmission of the human microbiome and demonstrated that household cohabitation is the main route for bacterial strain exchange. The team analyzed paired oral and gut metagenomes from 430 people across 207 households in Italy and Fiji, revealing how living together reshapes microbial communities independently of genetics.
The study overturns assumptions that family relationships alone explain shared microbiomes. Instead, people who share a home—whether they are related or not—exchange far more microbial strains with each other than with other members of their community. Notably, the strains most capable of moving between people are associated with markers of type 2 diabetes, poor cardiometabolic health, and microbes linked to colorectal cancer. These findings have direct implications for designing better microbiome-targeted therapies, including probiotics and fecal microbiota transplants (FMT).
Key findings
- Cohabitation, not heredity: Microbial strain sharing is driven primarily by shared environments. Siblings, parents, children and unrelated housemates showed similar levels of strain overlap, demonstrating that household exposure supersedes genetic relatedness as a determinant of strain-level similarity.
- Intimate contact accelerates oral transfer: Romantic partners showed a pronounced rise in oral strain sharing—averaging 44%—likely reflecting frequent close contact such as kissing. By contrast, the baseline oral sharing among general cohabitants was 26% and nearly zero for people in different households.
- Quantified household exchange: Across the cohort, cohabitants shared on average 19% of gut strains and 26% of oral strains. People living in separate households within the same community shared only about 6% of gut strains and effectively 0% of oral strains, establishing clear boundaries for routine interpersonal transmission.
- Host colonization filters: Although microbes move freely between people and environments, successful long-term colonization depends on a host’s tissue microenvironment. In other words, transmission is common, but establishment and persistence are controlled by whether the incoming strain can survive and adapt to the new host environment.
- Health-linked transmissibility: The gut species that showed the highest transmissibility correlated with biomarkers of type 2 diabetes and poorer cardiometabolic health. In the mouth, the most transmissible species included two taxa previously associated with colorectal cancer alongside several opportunistic pathogens that can be dangerous in immunocompromised hosts.
- Implications for therapeutic design: Mapping natural transmission traits provides a practical blueprint for improving microbiome therapies. By identifying phenotypic characteristics that enable certain strains to survive passage between humans, researchers can engineer probiotics and refine FMT protocols to improve colonization success and therapeutic outcomes.
Source: Cell Press
Overview: People who live together share more oral and gut microbes with one another than with others in their community, according to a study published June 15 in the Cell Press journal Cell Press Blue. This pattern held regardless of familial relationship—siblings, parents, offspring and unrelated housemates all shared similar strain counts, while romantic partners shared substantially more oral microbes, consistent with transmission via close contact and kissing.
The investigators also observed that highly transmissible microbes are often associated with adverse health markers, particularly those linked to type 2 diabetes and inflammation. These connections suggest that the traits promoting environmental resilience and person-to-person spread may also enable microbial success in inflamed or disease-prone tissues.
“Who we choose to live with can have a major impact on our microbiomes and, by extension, our health,” says Vitor Heidrich, first author and computational biologist at the University of Trento. The study fills a gap in understanding microbiome dynamics beyond infancy and clarifies how microbes move between body sites within the same individual, such as from the oral cavity to the gastrointestinal tract.
Senior author Nicola Segata, also a computational biologist at the University of Trento, emphasizes that diet and lifestyle shape the microbes already present in us, but do not fully explain where those microbes originate. This research directly traces their sources: the home environment and interpersonal contact.
The study analyzed 1,644 paired oral and fecal metagenomes from 430 participants in 207 households across Italy and Fiji. Strains were identified for each individual and compared across cohabitants to detect transmission. Cohabitants shared significantly more strains than non-cohabitants—19% of gut strains and 26% of oral strains on average—while people in different households shared only about 6% of gut strains and virtually no oral strains. Romantic partners shared an average of 44% of their oral strains.
Segata comments that it was unexpected how comparable oral and gut transmissibility appeared: “Most of our microbes are everywhere, and exchange is common, but long-term microbiome composition is shaped by whether our bodies accept colonization.”
Estimating transmissibility across species, the team linked highly transmissible gut bacteria to markers of type 2 diabetes and cardiometabolic risk. In oral communities, the top transmitters included two species associated with colorectal cancer and a suite of opportunistic pathogens. The authors suggest these traits may reflect stress resistance—attributes that help bacteria survive environmental exposure and then thrive in diseased, inflamed tissues.
The research offers concrete guidance for improving microbiome-based interventions. By mimicking the survival mechanisms of naturally transmissible strains, clinicians and pharmacologists can design next-generation probiotics and FMT approaches that better colonize recipients and deliver lasting benefit.
Key questions answered
Q: How does sharing a home reshape your internal microbiome even without genetic relatedness?
A: A shared household functions as a continuous, high-volume exchange network for microbes. People exchange microorganisms daily through shared air, surfaces, food handling, and direct contact. This constant exposure results in substantial strain sharing—about 19% of gut strains and 26% of oral strains among cohabitants—regardless of kinship, while people living apart share very little.
Q: Why are the most transmissible gut bacteria linked to diseases like type 2 diabetes and cancer?
A: The association likely reflects stress-resilient traits. Characteristics that enable bacteria to survive environmental challenges and transit between hosts—such as robustness to oxygen exposure or antimicrobial stresses—may also allow them to persist and expand within the inflamed tissues common to metabolic disease and cancer.
Q: How can clinicians use strain-tracking insights to improve probiotic and microbiome therapies?
A: Natural transmission rules can guide the engineering of therapeutic strains. Identifying the features that make some microbes highly transmissible enables researchers to select or modify beneficial strains with similar survival strategies, increasing the likelihood that therapeutic microbes will colonize and persist in patients receiving probiotics or FMT.
Editorial notes
- This article was edited by an editor at Neuroscience News.
- The underlying journal paper was reviewed in full.
- Additional contextual information was added by staff.
About this microbiome research news
Author: Julia Grimmett
Source: Cell Press
Contact: Julia Grimmett, Cell Press
Image credit: Neuroscience News
Original research (open access): “Strain transmission links human microbiomes along the oral-gut axis and across cohabiting individuals” by Vitor Heidrich et al., Cell Press Blue. DOI: 10.1016/j.cpblue.2026.100034
Abstract
Strain transmission links human microbiomes along the oral-gut axis and across cohabiting individuals
Interpersonal strain transmission shapes the human microbiome, yet comparative data across body sites have been limited. The authors analyzed 1,644 paired oral and fecal metagenomes to investigate transmission among healthy cohabitants and intra-individual oral-gut overlap. Cohabitants shared significantly more oral and gut strains than non-cohabitants. Romantic partners exhibited the highest oral strain-sharing rates, exceeding gut sharing. Higher oral transmissibility was associated with increased longitudinal strain replacement, while the most transmissible gut species correlated with poorer cardiometabolic health. Within individuals, 74.5% of species found at both oral and gut sites involved identical strains, largely driven by abundant oral species such as Streptococcus salivarius, consistent with saliva-mediated transfer. By contrast, Bifidobacterium longum strains did not overlap between sites, with a proposed B. longum subsp. unique to the oral cavity. These findings extend our understanding of microbiome spread and potential health consequences.