A new study from the University of Trento in Italy has found that sharing a home means sharing more than just space and chores—flatmates also exchange substantial parts of their gut and oral microbiomes. The research, published by a team led by computational biologists Vitor Heidrich and Nicola Segata, analysed 1,644 paired mouth and stool samples from healthy individuals to track how microbes move between people who live together.
The results show that cohabiting individuals share on average 19% of their gut microbiome strains and 26% of their oral microbiome strains. In contrast, people who do not live together share only 6% of gut microbes and none of their oral microbes. Romantic partners, however, stood out: they shared an average of 44% of their oral microbes, a figure the researchers attribute to kissing.
Microbial Exchange Beyond Family Ties
Importantly, the study found that the degree of microbial sharing was similar among siblings, parents, children, and unrelated flatmates. This suggests that daily contact and shared environments—such as preparing and eating meals together or using the same bathroom—drive the exchange, rather than genetic relatedness. “Who we decide to share our homes with can have a huge influence on our microbiomes, which has potential consequences for our health,” said Heidrich.
The human microbiome consists of trillions of microorganisms, including bacteria, viruses, fungi, and parasites. Each person’s microbial community is unique, shaped by factors like birth environment, infant feeding, long-term diet, and lifestyle. The new findings indicate that cohabitation adds another layer of influence, introducing new strains that can colonise the gut and mouth.
Segata noted that it was surprising the oral microbiome was only slightly more transmissible than the gut microbiome. “This speaks to the fact that most of our microbes are kind of everywhere, and the microbial exchange is very high, but our microbiomes are shaped more at the level of whether our body accepts the colonisation of these bacteria,” he explained.
Health Implications of Transmissible Microbes
The study also uncovered a concerning pattern: the microbes most easily transmitted between people were more likely associated with poor health outcomes, including type 2 diabetes and poor cardiometabolic health. In the mouth, the most transmissible species included two microbes linked to colorectal cancer and several opportunistic pathogens that can cause serious illness in people with weakened immune systems.
Heidrich speculated that these disease-associated microbes may have traits that help them survive the journey between hosts. “The same traits that help them survive the journey between humans may also allow them to thrive in the inflammatory conditions associated with disease,” he said. The findings raise questions about how shared living environments might influence the spread of health risks, though the researchers caution that correlation does not imply causation.
The research adds to a growing body of evidence on how social interactions shape our microbial ecosystems. For Europeans, where dense urban living and multi-generational households are common, the results could have particular relevance. As cities like Berlin, Paris, and Milan grapple with housing shortages and rising co-living arrangements, understanding the microbial consequences of shared spaces becomes more pressing.
While the study does not directly address interventions, it underscores the importance of considering the microbiome in public health discussions. For now, the message is clear: your flatmates are not just sharing your kitchen—they are also leaving a lasting microbial imprint on your body.
