The community of people living around you can influence the community of microbes living inside you.
The largest and most diverse review to date has found evidence that who you live with and who you were raised with can influence your microbiome more than certain lifestyle factors, age, or genetics.
If the findings are correct, the trillions of microbes that call our bodies home may be more contagious than we realize. And this can seriously affect public health.
The research, led by microbiologist Nicola Segata of the University of Trento in Italy, fails to show how bacteria jump directly from one person to another, instead illustrating how much our gut and mouth bacteria are shared with those around us.
Social interactions, the authors conclude, can help shape a person’s community of microbes, and thus “may play a role in microbiome-related 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 purposively 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 through brief encounters in public.
The bacteria strains shared between study participants were found to be ‘widespread’. In fact, the researchers identified more than 10 million instances of shared bacterial strains between mothers and infants, members of the same household, or people in the community.
Previous studies have shown that mothers help kickstart their babies’ microbiomes in the first few months of life by sharing their own microflora with them, usually through vaginal birth, breastfeeding, saliva exchange, or touch.
It is also known that a person’s microbiome can fluctuate throughout their life depending on what they eat, how much they exercise, or the environment they live in.
In comparison, human-to-human transmission has not been so extensively studied. The results of the current review suggest that this is an oversight.
As expected, mother-to-child transmission was the most important route of exposure. In the 711 cases, about 50 percent of the same bacterial strains were shared between mother and baby during the first year of life, and 16 percent of those strains came exclusively from the mother.
What’s more, this seed community of microbes can still be detected late into adulthood, albeit at a lower percentage. For example, by age 30, the average person in the study retained about 14 percent of their mother’s original bacterial strain. Even at age 85, the mother’s most transmissible strains were still present in her offspring.
As a person ages, the mother’s microbial influence is balanced by other relationships. Who a person lives with and interacts with on a daily basis appears to have an increasingly large effect on the makeup of their microbiome.
For example, after the age of four, researchers found that children share the same percentage of bacterial strains from both mothers and fathers. What’s more, the longer identical twins lived apart, they shared fewer microbial strains in their guts.
Overall, about 12 to 32 percent of the bacterial strains found in the gut and mouth are shared with others under the same roof. Similar lifestyle factors were not sufficient to explain the results.
“In adulthood, the sources of our microbiome are often the people we are in close contact with,” Segata explains.
“The duration of the interaction – think for example of students or partners sharing an apartment – is roughly proportional to the number of bacterial exchanges.”
When the authors turned to larger communities, they saw a similar, but smaller, relationship.
Less than one percent of bacterial strains appear to jump between households in the same rural community, making this a relatively rare form of transmission. That said, the transmissibility of bacterial species in rural communities was highly consistent across datasets.
In about 67 percent of the communities studied, people from the same village but from different households shared more bacterial strains than households from other villages.
The findings suggest that even superficial interactions can influence a person’s microbiome, for better or worse. While some microbes may come with health benefits, others can weaken the microbiome, making individuals susceptible to illness or disease.
“The transmission of the microbiome has important implications for our health, as some non-communicable diseases (such as heart disease, diabetes or cancer) are partially linked to an altered composition of the microbiome,” Segata explains.
“The demonstration that the human microbiome is transmissible may suggest that some of these diseases (currently considered non-infectious) may be, at least to a certain extent, infectious.”
The study was published in 2015 Nature.