Do couples share gut bacteria?

Course outline

Every lesson as plain text, if you'd rather read than click.

1. 1.Same song, same recordingBABY MODE

   Two strangers in different cities carry the same common kind of gut bacterium. What does that tell you? Very little. Many people carry the same species, so a matching species is weak evidence that anything passed between two people.

   Treat your gut like a long playlist. Each kind of bacterium, each species, is one song. Popular songs show up on lots of playlists, so a shared hit proves little. What would be strange is two playlists holding the very same recording: the same take, down to its tiny glitches. In biology that recording is a strain.

   Same-recording matches really do turn up in people who share a home. In a 2023 Nature study, of the gut species two housemates both carried, about 1 in 8 was the same strain (median 12%, our rounding). The rest were covers: same song, different band. Unlike a song file, though, nobody hits send, and these studies can't tell who a strain came from.

2. 2.What counts as the same strainBIG BABY MODE

   A strain match means two genomes of the same species are nearly identical. In the 2023 study, the cut-off was set so that no more than 5% of unrelated people would count as sharing a strain. It is a statistical call, and a strong one. In the mouth, housemates shared strains 10 times more often than non-housemates from the same population, while their species lists were less than 0.5 times more alike.

   Turning a rate into "1 in N" (our rounding)

   1. The rate = shared strains ÷ species that could be strain-profiled in both people. It is only calculated when the pair has at least ten such species.
   2. Housemates, gut: median 12%, so 1 ÷ 0.12 ≈ 8.3.
   3. Result: about 1 in 8 of the comparable shared species is the same strain. That is not 12% of the gut.

   Two housemates carry the very same strain. Can the study tell who gave it to whom? No. The studies can see the match, but they can't tell whether the strain passed directly or came from a common source, or which way it went. Observational data alone can't fully separate a shared environment from transmission.

3. 3.Home, mouth and distanceBIG BABY MODE

   In the gut, who shares more strains with you: your partner, or a sibling who lives with you? About the same. In the 2023 study, partners, parents and older kids, and siblings in one home didn't differ significantly in gut strain sharing.

   A separate 2025 study of 1,787 adults in 18 isolated Honduran villages found the same pattern: a median of 13.9% for spouses and 13.8% for people in the same household. Friends living in other homes shared more than unconnected neighbours, and the extra sharing reached friends of friends.

   The mouth is where partners pull ahead: a median of 38%, against about 13% in the gut. The authors think saliva can carry mouth bacteria directly. Distance cuts sharing fast: in the 2023 study, people from different villages or populations typically shared zero identical gut strains. Babies are the exception at home: they share far more with their mothers, 47% at one year.

   What happened to shared gut strains in twins who had moved apart? The longer twins had lived apart, the fewer they shared: from around 30% to about 10% after 30 years apart. These were different pairs at different stages. Identical twins still shared more than non-identical twins, a moderate genetic effect.

4. 4.Where "30%" comes fromBIG BABY MODE

   The headline "couples share 30% of their gut bacteria" traces back to a 2019 study of 408 older adults in Wisconsin. Spouses had about 30% of their bacterial types in common, siblings about 26%, and siblings were no more alike than unrelated pairs. Those types stand in for species, measured with a different method, so the 30% can't be set beside a strain rate.

   Couples eat together. Couldn't diet explain all of it? Not all of it. Eating the same food can make two guts look alike without any bacteria passing between people. But in the Wisconsin study the similarity held after accounting for diet, and the 2023 strain study removed strains matching fermented-food bacteria. Diet isn't the whole story.