The mouth and gut contain two of the richest microbial communities in the human body, and both are linked to health and disease. A new study now adds evidence to the idea that human microbiotas are not entirely personal: microbes move between close contacts and between the mouth and gut, with possible health consequences.

The findings, published in Cell Press Blue, suggest that who a person lives with—and how closely they interact—may help shape the microbes linked to health.

People who live together tend to share more microbiota strains than unrelated people, and oral microbes can sometimes appear in the gut. But it’s not well known how oral and gut transmission differ between close contacts and whether the most transmissible microbes have broader implications for health.

Vitor Heidrich at the University of Trento in Italy and his colleagues analyzed the microbiotas of more than 800 people to track microbial strains—nearly identical versions of microbial species that can reveal transmission across individuals.

Household microbes

People living together shared more oral and gut strains than unrelated people, with romantic partners showing especially high sharing of mouth microbes. This is consistent with close oral contact, including kissing, the researchers say. 

The mouth microbiota also showed more strain turnover over time than the gut microbiota, even though the broader makeup of both communities stayed stable. The mouth microbiota also had a larger share of highly transmissible species than the gut microbiota. 

Some of the most transmissible oral species included microbes that can act as opportunistic pathogens and oral species that have been associated with colorectal cancer.

Microbial transmission

Many of the highly transmissible gut species within households were associated with poorer health, including links to type 2 diabetes. However, this observation does not prove that these microbes cause disease, the researchers say.

When the team compared mouth and stool samples from the same people, only a small fraction of microbial species appeared in both places. But when the same species was found in both the mouth and gut, it was often the same strain. These shared strains tended to be abundant in the mouth and rare in stool, which suggests that many of them are oral microbes that reached the gut through swallowed saliva, the authors say.

The findings, they add, “delineate the power of high-resolution strain-sharing analysis in establishing a framework for understanding how human interactions and microbial features come together to dictate microbiome transmission and its consequences for human health.”