Some Gut Bacteria Can be Transmitted Through Close Social Contact

By Deborah Borfitz

December 11, 2024 | An intriguing new study suggests that the people we interact with socially influence the bacteria populating our gut. The implication is that some diseases considered nontransmissible may have a component that makes them contagious, according to Francesco Beghini, a postdoctoral associate in the Human Nature Lab of Nicholas Christakis, M.D., Ph.D. at Yale University. 

Inspiration for the investigation was a 2007 study in The New England Journal of Medicine (DOI: 10.1056/NEJMsa066082) finding that “obesity might spread by both social and biological contagion,” Beghini says. The odds of a person becoming obese increased significantly if they had a friend, sibling, or spouse who became obese, and the effect was more pronounced among persons of the same sex.  

“It is not just that people we are interacting with are influencing us with certain habits, but it is possible that biological components like the microbiome, in this case, may help facilitate the transmission of such conditions,” says Beghini. Microbial transmissibility needs to be taken into consideration when designing clinical trials and studies, since the social effect may impact outcomes.  

The phenomenon is “not necessarily a negative thing,” he adds, since beneficial components of the microbiome can also be transferable and may serve as a risk mitigator “by acting as a barrier against the colonization of certain bacteria.” Engaging in relationships with other people can also have a lot of mental health advantages.   

To study the potential association between social connections and the microbiome, researchers decided to comprehensively map the social networks of nearly 2,000 people living in 18 isolated villages in Honduras and perform detailed microbiome sequencing of each of their stool samples (Nature, DOI: 10.1038/s41586-024-08222-1). The individuals were previously enrolled in a different study and readily agreed to participate in this one, says Beghini. 

Mapping of the social relationships was done via questions such as who individuals spent their free time with and trusted to talk to about something personal or private. Researchers identified 4,658 unique social network links as well as details on the social encounters such as how often they happened, whether shared meals were involved, and how they greeted one another. 

Genetically distinctive strain-sharing between people suggests interpersonal transmission rather than common exposure to an environmental factor, they report. A state-of-the-art computational tool was used to perform metagenomic strain-level population genomics. 

‘Set of Niches’ 

Results were that connected people had more similar microbiomes and shared strains, and those similarities were stronger if they engaged in more frequent activities together, Beghini says. Investigators proposed neither a mechanism on how this phenomenon happens, nor which genetic components enable bacteria to spread. But based on their observations, the facilitator was the close contact between people. 

People who were connected through a variety of relationship types, including non-familial and non-household connections, exhibited similarities in their microbiomes that go beyond what would be expected through chance, Beghini says. Evidence of microbiome sharing was seen even after accounting for other factors such as diet, water sources, and medications. 

Based on strain-level modeling, microbiome sharing was shown to be “the strongest predictor of people’s social relationships in the villages... beyond characteristics like wealth, religion, or education,” he continues. The highest amount of microbial sharing occurred among spouses and people living in the same households, but the amount of time people spent together also had a disproportionate effect. 

A follow-up study was performed in an identical manner on a subset of 301 participants from four villages two years after the initial assessment, confirming that those who were socially connected had become more microbially similar that those who were not connected. Clusters of microbial species and strains were also discovered among groups of people within the different villages. 

Overall, the research team write, the social network structure of human populations appears to “provide a set of niches within which microbes can thrive or spread.” While they currently have no plans to further investigate this phenomenon, Beghini says it could be of enduring interest to those studying obesity, given that the microbiome plays a role in the disease. 

Evidence that gut microbiome information is socially transmissible has been building for at least a decade. Prior research cited in the latest study out of Yale’s Human Nature Lab found that the information can predict a host’s social interactions as well as be transferred between spouses and within households.