It’s not just milk, bills and a pile of washing up – you probably also share gut bacteria with your housemates.
That’s according to scientists from the University of East Anglia (UEA), who found living with friends or a partner may quietly be altering your gut bacteria.
“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 Chuen Zhang Lee, from UEA’s School of Biological Sciences.
Researchers studied a colony of tiny island birds and found they shared more of their gut bacteria with the birds they spend the most time with, and it is thought the same principle applies to humans too.
From birth, the gut microbiome – which consists of bacteria, viruses and fungi – is shaped by our surroundings, initially from our mother and later by our social interactions.
Previous studies on humans have hinted that who we live with has a huge influence on what microbes we have, even when eating different diets. Experts have suggested partners share around 30 per cent of their resident microbes in the gut alone.
But the new bird research provides unusually clear evidence of how social closeness itself, not just shared environment, drives the exchange of gut bacteria.
For the study, published in the journal Molecular Ecology, researchers collected data on the Seychelles warbler, a small songbird found on Cousin Island, by collecting faecal samples over several years from breeding pairs and other birds living in the group.
These samples were used to analyse the birds’ gut microbiomes: the diverse communities of “good” bacteria living in their digestive systems.
“This allowed us to compare the gut bacteria of birds that interacted closely at the nest versus those that did not,” Dr Lee said. “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.”
Because the birds never leave the island, scientists were able to track them over their lifetime and how close they were with other birds by fitting them with coloured leg rings.
“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.”
Researchers say a similar thing could be happening in human homes, whether it is with housemates or a spouse. But this could be improving immunity and digestive health.
“Anaerobic bacteria are some of the most important for digestion, immunity and overall health,” Dr Lee said.
“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 cosy 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.”
