Living Together Means Sharing More Than a Home, It Means Sharing Bacteria

People who live together share not only a kitchen and bathroom, but also much of their microbiome, according to a new study. Researchers from the University of Trento in Italy, publishing in Cell Press Blue, examined 1,644 mouth and gut samples from 808 participants in 207 households, and supplemented the findings with thousands of additional samples from earlier studies.

The study found that cohabiting people share, on average, 19% of gut bacterial species and 25.8% of oral species. Among couples, the overlap was even higher: 44.4% of mouth species and 19.5% of gut species. By comparison, people who do not live together shared only 6% of gut species and almost no oral species. The researchers said kissing and close contact likely explain much of the oral sharing, and first author Vitto Heidrich said household members can have a significant effect on the bacteria living in our bodies. Israeli gastroenterologist Dr. Nadav Ilani said the work shows that microbiomes are shaped not only by birth, diet, lifestyle, medications and genetics, but also by bacterial transfer among household members.

The team also found that the bacteria that spread most easily between people were not necessarily the beneficial ones. Highly transferable strains were often associated with poorer cardiometabolic health, including organisms previously linked to type 2 diabetes and other risk factors. The study also found two orally transmitted bacteria that had previously been linked to colorectal cancer, plus several opportunistic pathogens, but the authors stressed that the data show statistical association, not person-to-person transmission of disease or cancer.

Another part of the study showed that only 4% of bacterial species appeared in both the mouth and the gut, even though people swallow large amounts of oral bacteria every day. When the same species was found in both places in the same person, it was the exact same strain in 74.5% of cases, supporting the idea that some gut bacteria arrive via swallowed saliva. Over follow-up, 14.7% of oral species changed, compared with 5.8% in the gut, suggesting the mouth microbiome is more dynamic. The researchers said these findings could eventually help develop microbiome-based treatments, including probiotics and fecal transplants.