Summary: Researchers have mapped detailed environmental and biological factors that govern human microbiome transmission and demonstrated that household cohabitation is the principal route for strain-level bacterial transfer. The team compared oral and gut metagenomes from 430 individuals across 207 households in Italy and Fiji to quantify how microbes move between people and across body sites.
This analysis challenges assumptions that genetic relatedness is the main driver of shared microbiomes. People who live together—whether relatives or unrelated housemates—share substantially more microbial strains with each other than with others in the same community. The study also identified links between the strains most readily transmitted between people and biomarkers of cardiometabolic disease, including Type 2 diabetes, as well as oral species associated with colorectal cancer and opportunistic pathogens.
Key Facts
- Cohabitation outweighs genetics: The study shows that shared living environments explain strain-level microbiome similarities more than biological kinship. Siblings, parents, offspring and unrelated housemates show similar levels of strain sharing when they live together.
- Intimate contact increases oral exchange: Romantic partners share a notably higher portion of oral strains—likely driven by close contact such as kissing—compared with other cohabitants.
- Measured household exchange: On average, cohabiting individuals shared 19% of gut strains and 26% of oral strains. By contrast, people living in different households within the same community shared about 6% of gut strains and virtually no oral strains.
- Colonization depends on host environment: While transfer of microbes between people is frequent, whether those strains persist depends on the recipient’s tissue microenvironment and whether it permits colonization and long-term survival.
- Transmissible strains and health associations: The gut species with the highest transmissibility were linked to biomarkers of poorer cardiometabolic health and Type 2 diabetes. In the oral cavity, several highly transmissible species are associated with colorectal cancer risk and include opportunistic pathogens relevant for immunocompromised people.
- Implications for microbiome therapies: Understanding the traits that enable some strains to transmit and colonize effectively can inform the design of more successful probiotics and fecal microbiota transplant (FMT) protocols by replicating those survival characteristics for beneficial microbes.
Source: Cell Press
Key findings published June 15 in the Cell Press journal
People who share a home exchange more oral and gut microbes with each other than with others in their community. The effect is independent of family relationships: siblings, parents and children, and unrelated housemates who live together all show similar levels of microbial strain sharing. Romantic partners exhibited the highest oral strain overlap, consistent with close physical contact such as kissing.
The investigators examined paired oral and fecal metagenomes from 430 participants in 207 households across Italy and Fiji. They identified microbial strains in each person and compared strain presence and overlap across cohabitants and non-cohabitants to estimate transmission rates and patterns.
Average sharing rates were quantified: cohabitants shared about 19% of gut strains and 26% of oral strains, while individuals in separate households from the same population shared roughly 6% of gut strains and nearly 0% of oral strains. Romantic partners shared approximately 44% of oral strains on average.
The analysis also assessed which species are more transmissible. In the gut, the species with higher transmissibility correlated with clinical biomarkers of poorer cardiometabolic health, including Type 2 diabetes risk indicators. In the oral cavity, highly transmissible species included taxa previously associated with colorectal cancer and several opportunistic pathogens that can cause severe infections in vulnerable hosts.
Researchers propose that resilience traits—such as tolerance to environmental stress—may underlie both heightened transmissibility and the ability to thrive in inflamed or diseased tissues. Those same traits that help bacteria survive transit between hosts may also promote persistence in hosts with inflammatory conditions.
These findings have practical implications for therapeutic microbiome engineering. By identifying phenotypic and ecological properties that enable strains to transmit and colonize, scientists can better design probiotic strains and FMT preparations that establish more reliably in recipients.
Key Questions Answered:
A: Shared homes act as high-volume exchange hubs. Daily interactions—shared air, surfaces, food preparation, and physical contact—lead to continuous shedding and transfer of microbes. This study shows that such exposure produces substantial strain sharing across cohabitants, overriding genetic background in shaping which strains are present.
A: The association likely reflects survival traits. Strains that withstand environmental stress and the challenges of transmission (oxygen exposure, desiccation, competition) may also be better adapted to colonize inflamed or metabolically altered tissues. Those same features can correlate with disease-associated inflammatory states, though causality requires further study.
A: By treating natural human-to-human transmission as an engineering blueprint. Identifying characteristics that make some microbes highly transmissible—and understanding constraints that limit beneficial microbe transmission—can guide the development of therapeutic strains and transplant methods engineered for robust colonization and long-term persistence.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context 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: “Strain transmission links human microbiomes along the oral-gut axis and across cohabiting individuals” by Vitor Heidrich et al., published in Cell Press Blue. DOI: 10.1016/j.cpblue.2026.100034. Open access.
Abstract
Strain transmission links human microbiomes along the oral-gut axis and across cohabiting individuals
Interpersonal strain transmission is an important force shaping the human microbiome, but transmission dynamics across body sites have been less well characterized. The study analyzed 1,644 paired oral and fecal metagenomes to investigate microbiome transmission among healthy cohabitants and the degree of oral-gut overlap within individuals. Cohabitants shared significantly more oral and gut strains than non-cohabitants.
Romantic partners showed the highest oral strain-sharing rates, exceeding their gut strain sharing. Higher oral transmissibility was associated with increased longitudinal strain replacement. The most transmissible gut species were linked to markers of poorer cardiometabolic health. Within individuals, 74.5% of cases where the same species was detected in both oral and gut sites involved the same strains, largely driven by abundant oral species such as Streptococcus salivarius, suggesting saliva-mediated transmission. By contrast, Bifidobacterium longum strains did not overlap between sites, with the proposed B. longum subsp. nexti uniquely colonizing the oral cavity. These findings expand understanding of microbiome spread and potential health consequences.