Social networks and microbiome sharing

An article (Open Access! link) by a team led by Nicholas Christakis at Yale University reports studies of microbiome compositions from 18 isolated villages in Honduras and how they correlate with social interactions beyond just household members. Isolation means that individual microbiomes could reasonably be assumed to result solely from the environment, diet, medical conditions and treatments, and social interactions. The authors found a strong correlation between social networks and microbiomes up to second-degree connections, i.e., friends of your friends who are not also your friends share a good amount of microbiome.

Not surprising you might object. Perhaps. But it had to be done and the conditions for such a study plus the sheer amount of work needed to measure all the variables, plus accounting for health state, drugs, diet in analyzing the data make this a non-trivial result. In addition, the authors were able to predict social network interactions from microbiome composition. Just let that sink in!

One reason I find this result significant is going back to work by Christakis and his then-colleague James Fowler on the spread of what are usually considered non-communicable diseases, such as obesity (NEJM 2007, link) and depression (Mol. Psychiatry 2010, link). In this article, Christakis and his colleagues have found a plausible biological mechanism to explain the spread of these conditions (obviously there may be other mechanisms, biological, social including imitation, homophily,... ).

Now, correlation does not imply causation, and here, causation means "spread": there is no direct evidence of spread from one individual to another, although some indication of it based on strain-level identities.

See also Dr. Amine ZORGANI's post on this article: link

Upon reflection, I should have added a section about the social lives of dogs 🐕. Why? Because we know that a family's best friend shares his/her microbiome with said family! The New York Times even had an article titled "Are Pets the New Probiotic?" (link) or Nature Magazine's "Microbiome: Puppy Power" (link). More generally we share a lot of microbial organisms with our companion animals and even cattle (see my favorite rumen microbiome scientist Ben-Gurion University of the Negev's Itzik Mizrahi: link).

But back to dogs, as we can assume that village dogs spend a lot of time with people and are also free to roam. Assuming at least some degree of bidirectional transmission, humans <-> dogs and dogs<-> dogs, that could reinforce the correlation between people who share mutual human and canine friends. And what if two people that are more than 3 degrees apart (have no mutual friend) have dogs that hang out together?