The community of people living around you could influence the community of microbes living inside of you.
The largest and most diverse review to date has found evidence that who you live with and who you were raised by could have a greater impact on your microbiome than some lifestyle factors, age, or even genetics.
If the findings are right, then the trillions of microbes that call our bodies home could be more contagious than we've realized. And that could have serious implications for public health.
Social interactions, the authors conclude, could help shape an individual's community of microbes, and that, in turn, could "have a role in microbiome-associated diseases".
The findings are based on more than 9,000 stool and saliva samples collected from participants with known connections to each other. These communities were purposefully sampled from 20 diverse countries around the world, not just those in western or developing nations.
The findings strongly suggest that the trillions of symbiotic cells in our bodies can effectively spread between human hosts, even from brief encounters in public.
The strains of bacteria shared between participants in the study were found to be 'extensive'. In fact, researchers identified more than 10 million instances of bacterial strains shared between mothers and infants, members of the same house, or people in communities.
Previous studies have shown that a mother helps kickstart her kid's microbiome in the first few months of life by sharing some of her own microflora with them, usually via vaginal birth, breastfeeding, salivary exchange, or touch.
It's also known that a person's microbiome can fluctuate throughout their life based on what they eat, how much they exercise, or the environment in which they live.
By comparison, human to human transmission is hasn't been so extensively studied. The results of the current review suggest that is an oversight.
As expected, mother-to-infant transmission was the most significant route of exposure. In 711 cases, about 50 percent of the same bacterial strains were shared between mother and child in the first year of life, and 16 percent of those strains came from the mother specifically.
What's more, this seeded community of microorganisms could still be detected late into adulthood, albeit at lower percentages. At 30 years of age, for instance, the average person in the study had kept about 14 percent of their mother's original bacterial strains. Even at 85 years of age, a mother's most highly transmissible strains were still present in her offspring.
As a person ages, a mother's microbial influence is balanced by other relationships. Who a person lives with and interacts with on a daily basis seems to have an increasingly large impact on their microbiome's makeup.
After age four, for instance, researchers found a child shares equal percentages of bacterial strains from both the mother and their father. What's more, the longer identical twins lived apart, the fewer microbial strains they shared in their gut.
All in all, about 12 to 32 percent of bacterial strains found in the gut and mouth are shared with others under the same roof. Similar lifestyle factors were not enough to explain the results.
"In adulthood, the sources of our microbiomes are mostly the people we are in close contact with," explains Segata.
"The duration of interactions – think for example of students or partners sharing an apartment – is roughly proportional to the number of bacteria exchanged."
When the authors turned to larger communities, they noticed a similar, yet smaller, relationship.
Less than a percent of bacterial strains seemed to jump between households in the same rural community, making it a relatively rare form of transmission. That said, the transmissibility of bacterial species with rural communities was highly consistent across datasets.
In about 67 percent of the communities studied, individuals within the same village but from different households shared more bacterial strains than they did with households in other villages.
The findings suggest even superficial interactions can influence a person's microbiome, for better or worse. While some microbes might come with health benefits, others could weaken the microbiome, leaving individuals susceptible to illness or disease.
"The transmission of the microbiome has important implications for our health, since some non-communicable diseases (such as cardiovascular diseases, diabetes or cancer) are partly linked to an altered composition of the microbiome," Segata explains.
"The demonstration that the human microbiome is transmissible could suggest that some of these diseases (currently considered non-communicable) could, at least to a certain extent, be communicable."
The study was published in Nature.