Living Together Reshapes Oral and Gut Bacteria, Study Finds
A June 2026 study published in Cell Press Blue, conducted by researchers from the University of Trento in Italy with collaborators from Spain and Italy, reveals that cohabiting individuals share a significantly higher number of bacterial strains in their mouths and guts. Analyzing 1,644 paired samples of saliva and feces from 808 people, the scientists found that couples exhibited the most extensive sharing of oral microbiota among all family relationships studied. The research employed a genetic reconstruction tool called StrainPhlAn to identify specific bacterial strains, differentiating mere taxonomic coincidence from actual transmission events. The study found that cohabiting individuals shared a median of 19% of intestinal strains and 25.8% of oral strains, compared to only 6% and near zero, respectively, for individuals from different households. Notably, couples showed a higher proportion of shared oral bacteria (44.4% median) than intestinal bacteria (19.5% median), a pattern attributed to saliva exchange through kissing, which is considered a key factor in oral microbiome development in adulthood. Gastroenterologist Rogério Alves of Beneficência Portuguesa de São Paulo stated that this transmission is natural and not inherently risky, emphasizing that constant exchange occurs through air, food, and surfaces, with close proximity increasing transmissibility. He also noted that sharing utensils and poor oral hygiene can facilitate this exchange. The study further observed that oral bacteria are replaced more frequently (14.7% over 3.5 months) than gut bacteria (5.8%), suggesting the oral cavity's greater exposure to external environments. A significant finding is the correlation between transmissibility and health: intestinal bacteria most easily transmitted between cohabitants are often those linked to poorer metabolic profiles, such as Sellimonas intestinalis associated with type 2 diabetes markers, while beneficial bacteria are less frequently transmitted. This suggests disease-associated bacteria may have more efficient dissemination mechanisms. The research also identified a new subspecies of Bifidobacterium longum, tentatively named B. longum subsp. nexti, which appears to exclusively colonize the oral cavity.
This research highlights the dynamic nature of the human microbiome, demonstrating that intimate social connections, particularly cohabitation and romantic partnerships, significantly influence microbial composition through direct transmission. The study's findings challenge the notion of a purely individual microbiome, positing it as a shared ecosystem shaped by social proximity and behaviors like kissing. While the direct health risks of this bacterial exchange are deemed low by experts, the research opens avenues for understanding how transmitted microbes, especially those linked to metabolic diseases, might impact public health. Future investigations could explore the systemic implications of these microbial transmissions on population-level health outcomes, considering how environmental and social factors interact to shape both individual and collective microbiomes in an increasingly interconnected world. The discovery of novel oral bacterial subspecies also underscores the vast, unexplored diversity within human microbial communities and the potential for specialized adaptations to specific host environments.
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