Researchers at the University of Trento have shown that people who share a home also share a striking portion of the bacteria living in their guts and mouths.
The findings were published in June 2026 in Cell Press Blue.
The team tracked 430 adults across 207 households in Italy and Fiji, using genetic sequencing that can identify individual bacterial strains rather than just species.
People sharing a home matched on 19 percent of their gut microbiome strains and 26 percent of their oral strains.
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Adults in the same community who did not live together matched on only about 6 percent of gut strains and 0 percent of oral strains.
Romantic partners shared 44 percent of oral microbes on average, which the authors attribute to kissing.
Shared microbes
Not all of these shared microbes are neutral.
The team reported that the most easily transmitted gut bacteria overlapped with strains linked to type 2 diabetes and to poorer heart and metabolic health.
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Several highly transmissible mouth bacteria have been tied to colorectal cancer risk and to opportunistic infections, meaning infections that mostly threaten people with weakened immune systems.
That does not mean living with someone directly raises your disease risk, but it does suggest disease-associated microbes travel between people who share meals and a household.
Senior author Nicola Segata said the pattern shifts how researchers should think about lifestyle and the microbiome.
The new work looks instead at where those microbes come from in the first place.
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Another study
Separate work published in Nature, led by Xiaoqian Annie Yu and Martin F. Polz at the University of Vienna, adds another layer.
Their team examined thousands of bacterial isolates from multiple countries and found that many common gut species are actually made up of several genetically distinct populations.
Some of the most competitive populations have spread across continents within only a few decades.
Several of those populations correlate with advanced age, inflammatory bowel disease, colorectal cancer, and type 2 diabetes.
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What it could change in the clinic
If scientists can identify which microbial traits make bacteria good at spreading between people, they can build better probiotics and fecal microbiota transplants.
A fecal microbiota transplant is a treatment in which stool from a healthy donor is transferred to a patient with a damaged microbiome.
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This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.