Congratulations on moving in with your partner. You're now sharing a bathroom, a Netflix queue, and approximately one-fifth of each other's fecal bacteria. A study published this week in Cell Press Blue finds that cohabitating romantic partners share about 44% of their oral microbiome and 19% of their gut microbiome, and researchers are pretty sure at least some of that transfer is happening exactly the way you don't want to think about right now.
The Part Where We Explain What 'Sharing Gut Microbes' Actually Means
Look, there's no way to soften this. Vitor Heidrich, a computational biologist at the University of Trento in Italy and the study's lead author, told NPR directly: the gut microbiome sharing between housemates "entails that we are, to some extent, swallowing fecal matter from our housemates." He admits it is, quote, "difficult to grasp."
Yes. It is difficult to grasp. It is extremely difficult to grasp. But here we are.
The study analyzed microbiome DNA data from 430 people across 207 households in Italy and Fiji, making it one of the more comprehensive looks at how microbial communities travel between people who share a living space. The oral microbiome finding is especially notable because, as Cornell University biomedical engineering professor Ilana Brito told NPR, the oral microbiome is just harder to study. Getting clear signals from it is genuinely exciting to researchers in the field.
Kissing: Still Doing What You Thought It Was Doing
The oral microbiome numbers are striking. Romantic partners share about 44% of their oral microbes. General cohabitants, meaning roommates and family members, share about 26%. The gap between those two numbers has an obvious explanation, and Heidrich was not shy about stating it. He called the finding "a nice confirmation that when people exchange saliva directly, such as through kissing, you indeed see much more strain sharing."
For the less direct routes, Heidrich floated things like eating from the same dishes or toothbrushes touching in the bathroom. Which is either reassuring or makes you want to reorganize your entire bathroom cabinet, depending on your disposition.
The gut transmission finding is weirder, because it holds regardless of the relationship type. Romantic partners share roughly the same percentage of gut microbiome as regular housemates do, at about 19%. Which means the person subletting your spare room is also, biologically speaking, getting uncomfortably close to you.
Should You Be Worried? Scientists Say: Ask Again Later
Here's where it gets genuinely complicated, and where the study authors are careful not to overstate what they found. Some of the most transmissible microbial species the researchers identified have also been linked to higher risk of Type 2 diabetes in previous studies. Heidrich told NPR that connection is real, but he is also very clear about its limits: "We are only starting to investigate this as a possibility."
Dr. Jessica Queen, an infectious disease physician and assistant professor at Johns Hopkins University, told NPR the study raises important questions about whether your partner's microbiome health affects your disease risk. But she was equally clear that proving causation here is genuinely hard. "We have trillions of bacteria in our gut," she said, "and really trying to experimentally prove what is causative versus correlative, and what's the sequence of events, is actually very difficult."
Brito, whose own 2019 research on microbiome transmission among Fijians was among the data sources Heidrich's team used, put it plainly to NPR: "It might be the case that we protect each other; it might be the case that it has no substantial effect." All three researchers agree the field is years away from producing evidence strong enough for doctors to make lifestyle or treatment recommendations based on any of this.
The Researcher Who Had to Go Home to His Wife After Writing This Paper
In a detail that NPR correctly buried near the end but absolutely deserves more attention: Heidrich went through all of this research, concluded that cohabitating partners are definitely sharing microbes in ways that include some associated with disease risk, and then went home to his wife and two cats.
He says he isn't worried. His reasoning is actually solid: humans have been living together in groups for millions of years, and so did our primate ancestors. Swapping microbes with the people around us is, in his words, "intrinsic to the human experience." As for the cats, he's less concerned because pets have very different gastrointestinal environments, creating what he calls "bigger ecological barriers" to cross-species transmission.
So your partner's gut bacteria: probably fine, scientifically speaking. Your cat's gut bacteria: also probably fine. The fact that you are now thinking about all of this while looking at the person across the breakfast table: that one's on us, sorry.
The Dingo Take
Here's what's actually interesting about this study, underneath the obvious gross-out headline. We think of our bodies as our own sovereign territory, sealed units that belong only to us. Turns out that's never really been true. The microbiome research of the last decade keeps arriving at the same uncomfortable conclusion: we are permeable, we are communal, and the boundaries between one person's biology and another's are a lot blurrier than anyone's comfortable admitting. You are, in a very measurable sense, partly made of the people you've chosen to live with.
The honest scientific answer right now is that we don't know whether any of this is good, bad, or neutral for most people. The researchers who actually did this work are being admirably careful about not overclaiming. That's how science is supposed to work, and it's worth recognizing when it does. What we have is a well-constructed study that quantifies something real and raises questions that will take years of longitudinal research to answer.
In the meantime, the practical upshot is: nothing changes. You were already doing all of this. You were sharing microbes with everyone you kissed and everyone you lived with, long before anyone measured it. The study didn't create the phenomenon, it just described it. Whether that's horrifying or oddly comforting probably says something about how you feel about the people you've chosen to let into your home.
