How microbes travel between us and within us
Shared homes, shared strains: how microbes travel between us and within us
By Alena Pribyl, Lead Scientist at Microba
We often picture the microbiome as something unique, yet a large new study is a reminder of how readily our microbes move – between the people we live with, and between different parts of our own bodies. Drawing on more than 1,600 paired oral and stool samples from over 800 healthy adults, it tracked individual bacterial strains appearing in more than one place – a strong sign they had been passed along. Living together clearly mattered: people sharing a home shared a median of roughly 26% of their detectable oral strains and 19% of their gut strains, compared with essentially none of their oral strains and only about 6% of their gut strains among people from different households.
Broken down by relationship, that sharing concentrated in specific places. For the oral microbiome, romantic partners stood out, sharing more oral strains than any other household pairing (a median of around 44%) and – uniquely – more strains in the mouth than in the gut, which the authors link to saliva exchanged through kissing. For the gut, sharing was broadly similar across all close relationships, whether partners, siblings or parent and child.
Following strains within a single person, the team found that when the same species appeared in both the mouth and the gut, it was usually the very same strain – in around 75% of cases. This points to swallowed saliva as the route: oral species turning up in the gut is mainly explained by oral-to-gut transmission rather than each site acquiring its own version, with the most abundant oral strains the most likely to reach the gut at detectable levels.
The team also asked whether some gut species are simply better travellers – their transmissibility – and whether that matters. The most readily transmitted gut species were usually pathobionts: normally harmless residents that can tip towards harm under the right conditions. As a group they tended to be associated with poorer cardiometabolic health, and markers of type 2 diabetes were concentrated among them; Sellimonas intestinalis showed the strongest such link. Several health-associated microbes, by contrast, were more common in healthy people and spread less readily – hinting that beneficial species may be at an inherent disadvantage in passing between people.
These findings are worth watching rather than acting on. As an observational study of healthy adults, it describes associations and likely transmission but cannot prove cause or direction, and it detects only the most abundant strain per sample, so real sharing may be higher still. Even so, three messages stand out: cohabitants share far more strains than non-cohabitants; romantic partners show the highest oral transmission of any relationship; and the gut species that spread most easily tend to be those linked to poorer cardiometabolic health.
