Do couples share gut bacteria?

Yes, partly. In a 2023 Nature study, people sharing a home typically carried the very same strain in about 12% of the gut species they both had, roughly 1 in 8 by our rounding. 1 Most of each person's gut stays their own, and in the gut a partner matched about as much as parents, older kids and siblings at home. 1 2

The mouth is a different story. Partners who live together had about 13% in the gut but 38% in the mouth, and the study's authors think saliva is the likely reason. 1 People from different villages or populations usually share no identical gut strains at all. 1 What these studies can't tell is who gave a shared strain to whom, or whether both picked it up from a common source. 1 2

Your gut as a playlist

Think of everything living in your gut as a very long playlist. Each kind of bacterium, each species, is one song on it. Lots of people have the same popular songs, so two playlists sharing a hit tells you very little about whether those two people ever swapped music. 1

What would be strange is two playlists holding the very same recording of a song: the same take, down to the tiny glitches. That is what turns up in people who share a home. Of the songs two housemates both have, about 1 in 8 is the same recording. 1 The other 7 are covers: same song, different band. In biology the recording is called a strain, and two people carrying the same strain is the strong sign that bacteria passed between them, directly or indirectly. 1 2

The playlist is not small. A reference adult man of about 70 kg carries roughly 38 trillion bacteria, about as many as his own cells, not ten times more, and most of them live in the colon. 12

The analogy breaks in one place that matters. A song file gets copied on purpose, by someone who hits send. Bacteria don't get sent. And when scientists find the same strain in two people, they can't tell who it came from, or whether both picked it up from somewhere else. 1 2

Same species, same strain: how scientists tell them apart

The strain studies on this page use stool and saliva samples to compare people at two levels. 1 At the species level, most of us carry many of the same species, so matching species says little. At the strain level, researchers check whether one person's version of a species has a genome nearly identical to another person's. 1

A second comparison helps here. Two homework sheets with the same correct answer prove nothing, because anyone can know the answer. Two sheets with the same odd spelling mistake point to a shared source. A strain is that spelling mistake: a set of small genetic differences that unrelated people rarely have in common. 1 This picture breaks too. The same mistake could have been copied from a third sheet, and with bacteria, a shared meal or a shared home could be that third sheet. 1 2

"Nearly identical" has a precise meaning. In the 2023 study, the cut-off for calling two genomes the same strain was set so that no more than 5% of unrelated people would count as sharing it. 1 So "the same strain" is a statistical call, and a strong one, rather than a perfect copy.

The difference between the two levels is large. In the mouth, people who lived together shared strains 10 times more often than non-housemates from the same population, yet their species lists were less than 0.5 times more alike. 1 Comparing species would have hidden most of the sharing.

What the "12%" actually counts

The strain-sharing rate counts the strains two people share, divided by the species that could be strain-profiled in both of them. It is not a percentage of the whole gut. 1 2 The 2023 study only calculated it when two people had at least ten such species in common. 1

That is why "1 in 8" is the honest way to say it. A median of 12% means that, for a typical pair of housemates aged 4 and up, about one in every eight comparable shared species turned out to be the same strain. 1 For partners, about 13% works out to roughly 1 in 8 as well. 1

The study pooled 9,715 gut and saliva samples; the household comparison used part of them. 1

From the same home to the next village

Living under one roof makes the biggest difference. In the 2023 study, people in the same household shared a median of 12% in the gut and 32% in the mouth. 1 Without a shared home, sharing drops. Adults in the same village who didn't live together had a gut median of 8%, and people from different villages or populations had a median of 0%. 1 Those are village cohorts, so the 8% shouldn't be read as a figure for neighbours in a city. 1

In the mouth, people who didn't live together shared a median of 3% if they came from the same population and 0% if they didn't. 1

A separate 2025 study followed 1,787 adults in 18 isolated villages in Honduras. 2 Spouses had a median gut strain-sharing rate of 13.9% and people in the same household 13.8%, the highest of any relationship type. 2 Friends who lived in different homes shared more gut strains (median 7.8%) than unconnected people in the same village (4.0%) or people from other villages (2.0%), and the extra sharing reached friends of friends. 2 These are observations in isolated villages; they don't show which way any strain travelled. 2

Two separate studies, on different continents, landed on strikingly similar figures for partners: about 13% and 13.9%. 1 2 That is our comparison. The populations, methods and thresholds differ, so they shouldn't be read as the same number.

An earlier study of 287 people in Fiji found that strain sharing, in gut and mouth together, clustered most within households and between spouses. 13 The signal for any single pair was weak, yet strain data alone could pick out close to a quarter of the spouses with high confidence. 13

To your gut, your partner is one more housemate

This is the awkward part for a series called Science of Couples. In the 2023 study, every kind of family relationship in one home shared more gut strains than people from different homes, but partners, parents and kids, and siblings didn't differ significantly from each other. 1 Young siblings even shared slightly more than partners, though the difference wasn't significant. 1 The Honduran data point the same way: 13.9% for spouses, 13.8% for the household. 2

Partners do pull ahead in the mouth, where they had the highest median, 38%. 1 So to your gut, your partner is just another housemate. To your mouth, a slightly special one.

Babies are the big exception. Mothers and their babies share far more gut strains early on: medians of 65%, 50% and 47% at 1 day, 1 week and 1 year. 1 For offspring up to age 18 the figure was 19%, and up to age 30 it was 14%, close to the roughly 13% between partners. 1 That is why the comparison above is about older kids. These are different children at different ages, not the same children followed over time. 1

Why the mouth shares more

Mouth bacteria probably pass between people more easily than gut bacteria, since saliva can carry them directly. 1 People who live together share far more mouth strains than people who don't (a median of 32% vs 3% in the same population), and the longer they've lived together, the more they share. 1 Between partners, mouth strains were shared almost three times as often as gut strains, by our comparison of 38% and about 13%. 1

There is another clue in the bacteria themselves. In the 2023 study, fewer than 10% of the gut species tracked were predicted to survive oxygen, versus more than two-thirds of mouth species. 1 Those are predictions made from the genomes, and the authors don't present them as the cause of the gap. 1

Kissing comes up here, but it explains less than you might think. A small 2014 study of 21 couples estimated that one 10-second kiss moves about 80 million mouth bacteria, yet a single kiss didn't make partners' mouth bacteria measurably more alike than they already were. 6 Couples who kissed often did have more similar saliva. 6 None of that tells us anything about the gut.

How do gut bacteria get from one person to another?

These studies haven't pinned this down. The studies can see the same strain in two people, but they can't tell whether it passed directly or came from a common source, or who gave it to whom. 1 2 The authors of the Honduran study write that observational data alone can't fully separate a shared environment from transmission. 2

Food is the obvious suspect. Eating the same food can make two guts look alike even if no bacteria pass between people. 2 The 2023 study removed strains that matched known fermented-food bacteria, so they wouldn't count as sharing. 1 That filter only covers commercial fermented foods, and the authors don't rule out other food sources. 1

Spores are one more piece of the puzzle. A 2016 study estimated that at least half of the bacterial kinds (genera) in a healthy gut can make tough spores, which the authors say are built for moving from one person to another. 11 That study didn't look at couples, so it is a possible route, not a measured one.

Time, distance and genes

Twins give the clearest view of time. The longer twins had lived apart, the fewer gut strains they shared, on average: from around 30% down to about 10% after 30 years apart. 1 Those are different twin pairs at different stages, not the same twins followed for 30 years. 1 No study here followed couples after they split up.

Genes still count for something. Identical twins kept sharing more strains than non-identical twins decades after they stopped living together, which the authors call a moderate genetic effect. 1 Even so, the time since people last lived together affected strain sharing more than age or genetics did. 1 A 2018 study came at it from a different angle: unrelated people sharing a household have noticeably similar gut microbes, and host genetics play a minor role in the overall mix. 3

Sharing also stays partial because your own strains tend to stay put. In 37 adults followed for up to five years, most gut strains stayed, and a model suggests many could last decades. 7 Another study found that each person's strains are not easily replaced. 8

Where the "30%" headline comes from

You may have seen the line "couples share 30% of their gut bacteria". 5 The article behind that headline points to a 2019 study of 408 older adults in Wisconsin. 5 4 There, spouses had more gut bacterial types in common than siblings (about 30% vs 26%), and siblings were no more alike than unrelated pairs. 4 The gap between couples and unrelated pairs came from couples who said they were very close, and the similarity held after the researchers accounted for diet. 4

Those bacterial types are a stand-in for species, measured with a different method. The 30% says nothing about identical strains, and it isn't 30% of anyone's gut. 4 It shows a link, not a cause. 4 It also can't be compared with the strain figures of 12% or 13% on this page. 1 4

The question people ask next, whether sharing is good or bad for you, has no answer yet. What sharing means for health is still unknown. 9