Living with our significant other can modify our microbiome, researchers find, but only to a certain extent. Ultimately, our microbial profiles tend to keep their own "imprint," despite the fact that bacteria hop on and off.

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Cohabitating couples exchange bacteria, but their microbiomes remain individualized, according to a new study.

A team of researchers based at the University of Waterloo in Ontario, Canada, recently set out to conduct a study aiming to test whether couples that live together exchange microbes to a significant extent.

Their findings were mixed: cohabitating couples do swap bacteria to a notable degree, but not so much as to modify each other's microbial profile in a meaningful way, reports lead author Ashley Ross in the journal mSystems.

"Can we link couples back together? The answer is yes, but not a very loud yes," says senior study author Dr. Josh Neufeld.

Biological sex versus cohabitation

The study worked with 330 skin swabs collected from 17 different spots on the bodies of the participants, including their upper eyelids, inner and outer nostrils, torso, inner thighs, the back, armpits, navel, palms, and feet. There were 20 participants, all of whom heterosexual and lived in the same geographical area.

After testing the samples, the researchers noted that microbes had been swapped between couples to a significant enough extent that computer algorithms were able to link a person to their partner with an accuracy of 86 percent.

While this is no mean feat, Ross and her teammates also remarked that, despite this outcome, other factors are more responsible for shaping an individual's microbial profile.

Swabs of skin from different parts of the body told slightly different stories about a person's microbiome. For instance, the most similarities between microbiomes were found on cohabitating partners' feet.

This, the researchers explained, may be due to the fact that couples walk barefoot on the same surfaces in their house, thus easily exchanging bacteria with each other and their immediate environment.

"You shower and walk on the same floor barefoot. This process likely serves as a form of microbial exchange with your partner, and also with your home itself," explains Dr. Neufeld.

One of the most influential factors for a person's microbial profile, the study found, is biological sex. The bacterial diversity on the inner thighs of people of the same sex showed more similarities compared with the samples taken from the same body parts of cohabitating partners.

Computer algorithms were able to differentiate with a 100 percent degree of accuracy between people of different sexes just by analyzing samples of skin from their inner thighs. However, swabs from other body parts did not yield the same perfectly accurate results, though biological sex could still be ascertained 80 percent of the time.

Other findings included the fact that a person's left side is very similar to a person's right side in terms of the microbiome. This means that bacteriologically, we are fairly symmetrical.

The researchers also noted that swabs from the nose held the least bacterial diversity.

Impact of lifestyle choices

Factors such as the participants' personal hygiene, ownership of pets, existing allergies, and drinking habits were also taken into account to test for potential impact on the microbiome.

It was found that individuals who liked to drink alcohol several times per day had less diverse microbiomes. At the same time, people who spent more time outdoors and had a pet were likely to exhibit more bacterial diversity.

Also, individuals who reported living with many types of allergies had a more diverse microbial profile in both the inner and outer nose area.

The researchers acknowledge that the small population sample was a potential limitation to their study, and they express the need to further test their results using a larger population.

In addition, they explain, "Future skin microbiome studies should include same-sex couples to answer intriguing questions about how intimately living with a member of the same sex affects the microbiome."