The people you live with could be changing your gut bacteria

A study of small island birds found that individuals share more gut microbes with those they interact with most often. Researchers say this same effect is very likely happening in humans as well.

Earlier studies in people have suggested a similar pattern. Couples and long-term housemates tend to have more alike gut microbiomes than unrelated individuals, even when their diets are not the same. The new findings provide stronger evidence that close social contact itself, rather than just a shared environment, plays a key role in how gut bacteria are exchanged.

Study shows social contact shapes gut bacteria

The research focused on the Seychelles warbler, a small songbird that lives on Cousin Island in the Seychelles.

Scientists collected fecal samples from the birds and used them to study their gut microbiomes, which are the communities of beneficial bacteria in the digestive system.

Dr. Chuen Zhang Lee from UEA's School of Biological Sciences conducted the study as part of his PhD.

He said: "To uncover how gut bacteria spreads between social partners, we meticulously collected the birds' poo over several years. We gathered hundreds of samples from birds with known social roles -- breeding pairs, helpers and non-helpers living in the same group, and in different groups.

"This allowed us to compare the gut bacteria of birds that interacted closely at the nest versus those that did not.

"We studied their anaerobic gut bacteria, which thrive without oxygen.

"And it gave us a rare insight into how social bonds can drive the transmission of gut microbes."

Island setting offers unique research conditions

Cousin Island provided an ideal natural setting for this long-term study.

Senior researcher Prof David S Richardson explained: "Cousin Island is small, isolated, and the warblers never leave it. That means every bird on the island can be individually marked and followed throughout its life.

"This offers scientists an exceptional opportunity to study life-long biological processes in the wild."

Each bird is fitted with colored leg rings, allowing researchers to monitor behavior, health, and genetics over many years. This creates conditions similar to a controlled laboratory population while still reflecting real-world environments.

"It gives us the best of both worlds," said Prof Richardson. "We can study animals living natural lives, with natural diets and gut bacteria, while still being able to collect detailed data from known individuals."

Close relationships drive microbe sharing

The results showed a clear pattern. Birds that spent more time together had more similar gut bacteria, especially anaerobic microbes that can only survive in low-oxygen conditions.

"We found that the more social you are with another individual, the more you share similar anaerobic gut bacteria," said Dr. Lee.

"Birds who spent a lot of time together at the nest -- breeding couples and their devoted helpers -- shared a lot of this type of gut bacteria, which can only spread through direct, close contact.

"These anaerobic microbes can't survive in the open air, so they don't drift around in the environment. Instead, they move between individuals through intimate interactions and shared nests."

What this could mean for human gut health

The researchers believe these findings highlight what may be happening in human households.

"Whether you're living with a partner, housemate, or family, your daily interactions -- from hugging, kissing and sharing food prep spaces -- may encourage the exchange of gut microbes," said Dr. Lee.

"Anaerobic bacteria are some of the most important for digestion, immunity and overall health. Once inside the gut, they thrive in oxygen-free conditions and often form stable, long-term colonies. That means the people you live with might subtly shape the microscopic ecosystem inside you.

"Translated into human terms, this means that cozy nights in, shared washing-up duties, and even sitting close on the sofa may bring your microbiomes quietly closer together.

"Sharing beneficial anaerobic bacteria could strengthen immunity and improve digestive health across a household," he added.

The study was led by UEA in collaboration with researchers from Norwich Research Park, including the Centre for Microbial Interactions, the Quadram Institute, and the Earlham Institute, along with the University of Sheffield, the University of Groningen (The Netherlands), and Nature Seychelles.

The findings are published in the journal Molecular Ecology in a paper titled 'Social structure and interactions differentially shape aerotolerant and anaerobic gut microbiomes in a cooperative breeding species.'