Even if you’re not swapping spit, you’re definitely swapping microorganisms.

Researchers in a new study found that people who lived together shared 26% of the same oral microbes. This was regardless of the relationship — participants included siblings, parents and children. Romantic partners shared even more oral microbes (44%), likely due to kissing.

“Your microbiome is not just yours as an isolated entity, it is partly a reflection of the people you live with, and theirs is partly a reflection of you,” lead author Vitor Heidrich told The Post.

“This also means that the health benefits and disease risks linked to specific microbiome members may themselves be transferable between people, which is something we are only beginning to understand.”

Microbes are a diverse range of bacteria and other microscopic organisms, both beneficial and harmful, that are too small to be seen without a microscope. Generally, the more diverse these microbes are, the better suited the environment (the microbiome) will be to disturbances, and better for your health.

Eating the same foods and lifestyle habits, like being physically active, have important effects on the microbiome in the gut and mouth. But in terms of specific shared microbes, these factors aren’t actually as significant compared to living together, Heidrich said.

“Unless my strains physically travel from me to you, the same diet alone will not necessarily make us share more of our strains,” said Heidrich, a researcher at the University of Trento, Italy.

“It’s more that if I pass one of my strains to you and we eat the same diet, that strain will find a similar nutritional environment in your gut to the one it was thriving in before, making it more likely to successfully colonize your gut.”

How far microbes can go — and the oral-gut microbiome axis

If you’re passing microbes between others that live with you, what about others in your community? The answer is yes — that too.

“People living in different households but from the same community, like the same town, do share more strains than people from completely different populations,” Heidrich said.

“This suggests that microbiome transmission extends beyond the household, likely through untraced social interactions, shared public spaces, or strains circulating in the local community. However, the effect is much weaker than what we see within households.”

Within the individual body, researchers also saw similarities between the microbiome of the the mouth and gut, the body’s two largest microbial ecosystems.

Most of this can be explained by the simple fact that we swallow saliva all the time, transferring microbes in the saliva to our gut, Heidrich explains.

“This speaks to the fact that most of our microbes are kind of everywhere, and the microbial exchange is very high,” said Nicola Segata, another author from the University of Trento, in a release for the study.

Researchers call the connection between the two systems the oral-gut microbiome axis. Experts believe that knowing more about the interplay between the oral and gut biomes can help us better understand and treat gastrointestinal diseases and cancer.

The future of treating disease?

Authors of this paper also hope their research can help treat disease.

By identifying the microbes that were most transmissible between people, they can identify the traits that made them able to survive the journey.

“The same traits … may also allow them to thrive in the inflammatory conditions associated with disease,” Heidrich said.

This could help improve a treatment called fecal microbiota transplantation, where a sample containing a healthy microbiome is implanted in a sick person. This is a common treatment for an infection called C. diff, which can be resistant to antibiotics.

“If we can identify the characteristics that makes some microbes more transmissible than others, and the constraints that make beneficial microbes less transmissible, we can apply that to make fecal microbiota transplants much more effective,” Heidrich said.

It may seem freaky that just living together can cause such a vigorous and invisible exchange. But Heidrich reminds us there’s actually nothing more natural.

“Humans and our primate relatives have always lived in groups, so this kind of microbial exchange has likely been happening for millions of years, shaping the evolution of our microbiomes,” he said. “It’s a mechanism intrinsic to the condition of being human and that we should not be afraid of.”