Tell me what bacteria live in your gut and I’ll tell you who your friends are

Mencius, the ancient Chinese philosopher, wrote centuries ago that “friendship is one mind in two bodies.” Modern science might add another layer: friendship is also one microbiota in two bodies. A recent study has found that the more people interact, the more similar the composition of the microorganisms in their guts becomes, even if they do not live in the same household. Published in Nature, the research also reveals that an individual’s microbiome is influenced not only by their closest social contacts but also by the connections of these contacts — in other words, the friends of their friends.

To understand the details of this research, we must venture to the heart of the western Honduran jungle. It was here, in this Caribbean country, that scientists from Yale University spent 10 years recruiting 1,787 adults from 18 isolated villages to donate fecal samples. All participants followed a traditional diet and had limited exposure to antibiotics or other medications. Nicholas Christakis, the study’s lead author, explains that the team was “very lucky that the participants were helpful and committed.” The researchers needed to trace each volunteer’s contacts with precision, something that would have been far more challenging in cities like Madrid or New York. The villages in Honduras, in this case, were ideal.

Before delving into the results of this research, it’s important to explain what the microbiota is and why it matters. Francisco Guarner, director of the Digestive System Research Unit at Vall d’Hebron General Hospital in Barcelona, offers this definition: “It is the community of bacteria, viruses, and fungi that colonize the digestive tract. We could think of it as another organ in the human body — a set of biological functions that aid in the survival of an individual.” Though this “organ” resides within us, it operates according to its own rules and hierarchies, organized in its own way, and is not easily manipulated.

“It is essential for the digestion of food. It provides us with enzymes and metabolic pathways that humans lack,” explains Guarner. Thanks to the microbiota, for example, we are able to digest fiber. It also plays a crucial role in developing a balanced immune system.

For decades, science has explored the composition of the microbiota to understand how it develops in each person. Mireia Vallés Colomer, director of the Microbiome Research Group at Pompeu Fabra University, explains that vertical transmission was previously considered the most likely explanation. “We inherit these microorganisms largely from our mothers, through childbirth and breastfeeding. We also share bacteria that our grandmother passed to our mother,” she says. However, the new study suggests that the microbiota evolves throughout life, with social contacts playing a major role in these changes — what is known as horizontal transmission.

“We were really surprised by the extent of microbial sharing among social networks. We can actually predict who your friends are based on how similar the microbes in your stool are to theirs,” says Christakis. The data suggest that individuals living in the same household share up to 14% of the microbial strains in their guts. Those who don’t live together but spend considerable time together share 10%. The study also found that individuals living in the same town but with limited interaction share only 4%. The authors note that there is a clear chain of transmission, as friends of friends share more microbial strains than would be expected by chance alone.

The method of transmission

One lingering question surrounding this research is how bacterial strains are transmitted from one microbiota to another. “We don’t have a conclusive answer as to how this transmission occurs,” says Vallés. “What is hypothesized is that what reaches the intestine must pass through the mouth. Many bacteria in the microbiome do not tolerate exposure to oxygen for very long, so close contact is necessary, but we do not know exactly how this happens.”

However, Guarner explains that “the fecal-oral route seems to be the most important transmission vector.” In other words, although we maintain varying levels of personal hygiene, previous studies have shown that bacteria typically found in the intestine also appear on the hands. This is how they eventually make their way to the mouth.

Some bacteria manage to survive the journey from the intestine by traveling in the form of spores, similar to fungal spores. “With this transmission mechanism, direct contact doesn’t have to be extremely close — it can occur through a towel or clothing,” explains Guarner.

There’s no need to panic. This transfer of microorganisms is, in many ways, essential to our survival. In fact, new lines of research on the relationship between the microbiota and health suggest that a healthy, balanced community of microorganisms has a significant impact on various aspects of our well-being.

Some researchers are attempting to establish a direct link between the microbiota and non-communicable diseases, such as cardiovascular diseases, diabetes, and even depression. Guarner explains: “This is still somewhat speculative, but what is commonly observed is that these diseases are associated with a deficient microbiome.” Vallés adds: “It has been observed that individuals with so-called ‘modern diseases’ suffer an alteration in the composition of their microbiome.”

However, it is not that a specific bacterium is responsible for these diseases; rather, it is the overall loss of diversity within the microbiome that exacerbates health issues. In this context, the research opens the door to further exploration of whether these non-communicable diseases indeed have a transmissibility factor. If an entire community has a weakened microbiota, these diseases could potentially spread more easily among that population.

“We already knew that the social environment has a very important impact on general health. Now we see that it also helps shape our microbiome, and this is something we can no longer ignore. We cannot study humans as if they lived in isolation, but rather we must do so together with the community they are part of,” Vallés concludes.

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