Live with a partner? You may be sharing more microbes than you think

First author and computational biologist Vitor Heidrich of the University of Trento, Italy says that his lab was investigating potential sources of the microbes inside us "because before birth we don't have a microbiome, so they must be coming from somewhere."

Previous evidence has existed that people who live together share microbiomes. But the new study — which analyzed microbiome DNA data of 430 people across 207 households in Italy and Fiji — quantifies the transmission rate by relationship and includes the oral microbiome as well.

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"It's exciting — the oral microbiome is just harder to study, so it's exciting that they're able to pick up these signals," says Ilana Brito, associate professor in biomedical engineering at Cornell University and an expert on microbiome transmission. She was not directly involved with this study but her 2019 study on microbiome transmission among Fijians was among the data sources Heidrich used for his study.

As for how the microbe strains were transmitted, Heidrich has a few ideas. It could be from eating from the same dishes or toothbrushes touching in the bathroom, for example. The research team found that romantic partners on average share more of their oral microbiome (44%) than cohabitants overall did (26%).

Heidrich calls it "a nice confirmation … that when people exchange saliva directly, such as through kissing, you indeed see much more strain sharing."

Meanwhile, the study found cohabitants share approximately 19% of their gut microbiome regardless of their relationships.

"It's difficult to grasp the idea that we are sharing [gut microbes]," Heidrich admits, "because this entails that we are, to some extent, swallowing fecal matter from our housemates." It is likely that we are, he says.

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"Even a single microbial cell can be enough for a successful transmission event," he adds.

The vast majority of bacteria are harmless or beneficial, even fecal ones; some experts estimate that only about one in a billion bacterial species are human pathogens. But Heidrich says the researchers observed that some of the transmissible microbes were associated with poor health. In previous studies, some of these bacteria were found to be more common in patients with certain diseases.

Specifically, "we found that a lot of the very transmissible species are also linked with higher risk of Type 2 diabetes," he says.

Dr. Jessica Queen, an infectious disease doctor and assistant professor of medicine at Johns Hopkins University, says the study raises some questions: "Does the microbiome health of your partner, or your immediate family who you live with impact your disease risk?"

"I think we are very far from making this kind of claim," Heidrich says. "We are only starting to investigate this as a possibility."

Queen, who was not involved in the study, says there is likely a bi-directional relationship between the microbiome and diseases — our microbiome may affect disease risk, but a disease could also affect the mix of microbes in our bodies.

"We have trillions of bacteria in our gut, and really trying to experimentally prove what is causative versus correlative, and what's the sequence of events, is actually very difficult," she says.

The microbiome is known to affect our immune system, digestion and even potentially our brains.

None of this has stopped Heidrich from thinking about who he shares his home with, however. Heidrich lives with his wife and two cats, and he admits they are probably contributing to his microbiome to some extent. He isn't too worried when it comes to pets as there are "bigger ecological barriers" to crossing over because pets have very different gastrointestinal tract environments.

As for living with his wife, he says he isn't concerned either. He says, "as humans, we've lived together, sometimes in big groups, for millions of years, and [so did] our primate relatives." He thinks exchanging microbes with the people around us is intrinsic to the human experience even if there could be some consequences.

The field is likely years away from providing evidence that could lead to doctors making lifestyle or treatment recommendations for enhancing our microbiomes, Queen says. Heidrich and Brito agree. Queen says more experiments are needed, including looking at more long-term longitudinal data and identifying causal relationships in animal models with specific bacterial strains.

Brito says, "it might be the case that we protect each other; it might be the case that it has no substantial effect." She says she doesn't think there's a reason for people to stop embracing or otherwise exchanging microbes.