Cohabitants Share One Quarter of Gut and Oral Microbes

A new study has found that people who live together share a significant portion of their internal bacteria, with romantic partners exchanging mouth microbes at a particularly high rate. The research, published on June 15, 2026, in the journal Cell Press Blue, provides a detailed map of how bacteria move between individuals and within the body, suggesting our microbiomes are continuously reshaped by those we share a home with.

Scientists from the University of Trento in Italy analyzed genetic data from saliva and stool samples of 430 people across 207 households in Italy and Fiji. By tracking individual bacterial strains, they found that cohabitants shared approximately 19% of their gut microbial strains and 26% of their oral strains, compared to just 6% and 0% among people living in separate households in the same communities.

Romantic partners stood out for oral bacteria sharing, with a median rate of 44.4%. Researchers attribute this to more direct physical contact, such as kissing, which provides a clear pathway for mouth microbes. In contrast, gut bacteria sharing was roughly similar across all household relationships, including parent-child and sibling pairs.