Couples share 30% of their gut bacteria. Here’s how that may affect health

When living with a partner, you might be sharing more than just the same home, lifestyle and interests. You might also share various microscopic organisms residing on and in you.

This community of microorganisms, which consists of mainly bacteria, viruses and fungi, is known collectively as the human microbiome. The various microbiomes found throughout the body all play an important role in health.

From birth, the human microbiome is shaped by our interactions with our mother, who introduces diverse microorganisms that build our immune and digestive systems. As we get older, social interactions with our close community continue influencing this delicate ecosystem.

The people we live with have huge influence on what microbes we have in our microbiome. In fact, it’s thought that partners share around 30% of their resident microbes in the gut alone.

But it isn’t just the microbes in your gut that may be similar to your partner. The microbes in many other parts of the body may also be shared with your loved one – and this could potentially affect your health.

Gut microbiome

Diet and lifestyle are thought to have the greatest influence on the gut microbiome’s make-up. But studies on couples have found that living with your partner can also influence the microbiome.

Couples living together may share 13% to 30% of their gut bacteria. This was true even when diet (which many couples share) was factored out. Research also shows that couples who live together have greater microbial diversity compared to people who live alone.

This is good news for couples who co-habitate, as a more diverse gut microbiome is correlated with lower risk of irritable bowel syndrome, cardiovascular diseases and potentially high blood sugar.

But it might not all be good news. Research shows that some of the bacterial species couples share can have varying effects on health.

Take the bacteria from the Ruminococcus family. While some species of Ruminoccocus benefit health, others have been linked to negative health outcomes, including diabetes and irritable bowel syndrome.

So these bacteria may not always offer the same benefits in different demographics. This highlights the complexity of resident gut bacteria and their health impacts.

Oral microbiome

Sharing an oral microbiome with our partners might seem obvious considering we regularly exchange saliva when we kiss. A ten-second kiss alone can exchange up to 80 million bacteria. The more kisses a couple shares, the more shared salivary bacteria they will have.

Although most of these bacteria will quickly pass through our mouth and into our gut when we swallow saliva, research show that couples actually share many of the same longer-term tongue microbes that form the foundation of the oral microbiome. Research even suggests that 38% of the oral microbiome is shared in couples living together – compared to only 3% in couples who don’t live together.

Sharing this proportion of your oral microbiome could have many potential health effects.

A healthy oral microbiome is important for protecting against tooth decay and it has anti-inflammatory properties. Some researchers also suggest the oral microbiome’s health effects may extend as far as the gut and nervous system.

But some of the bacteria that couples tend to share may also have potentially harmful health effects.

Couples are more likely to have similar numbers of the bacteria Neisseria in their gut compared to single people. Neisseria can reside in the mouth for long periods of without causing disease.

Some Neisseria bacteria can be harmful and may cause meningitis. Yet some Neisseria bacteria actually fight against these meningitis-causing species, stopping them from overgrowing and causing harm.

So while you may want to avoid kissing someone when they’re poorly for obvious reasons, it turns out that a kiss even when you’re healthy can transfer all sorts of bacteria between the two of you.

More research is needed to really understand what overall effect sharing these bacteria with your partner has on health.

Skin microbiome

The skin microbiome is the most unique and personalised microbiome, tailored to each person. It’s even sometimes referred to as our microbial fingerprint.

Being the most exposed microbiome, the skin microbiome has evolved to be adaptable to external factors such as the climate and cosmetic products. No matter what, these bacteria work hard to remain at an equilibrium.

Close contact with our partners – and even pets – has a huge influence on what bacteria live on our skin. After comparing the gut and oral microbiome, researchers found the skin microbiome to be the most similar among couples.

It isn’t just the bacteria on your arms or hands that are shared, either. Research shows that couples shared 35% of the bacteria living on their feet, and around 17.5% of the bacteria on their eyelids.

You may not even need to touch your partner to have the same skin bacteria as them. Factors such as sleeping in the same bed and walking on similar surfaces are thought to explain why such a large proportion of our skin microbiome is similar.

This is because humans naturally shed bacteria in a similar way as dogs shed fur. We leave traces of our bacteria on everything we touch – and we also easily pick up bacteria from our environments.

The shared effect of living together on the skin microbiome is so great that researchers were able to use computer models to accurately predict 86% of cohabiting couples based off of their individual bacterial samples alone.

But while it’s clear that couples share much of the same skin microbiome, the health effect that this has is not currently known.

While sharing bacteria with your partner may sound alarming, there’s often no cause for concern. Bacteria teach our bodies how to fight infections, they help us digest foods and even produce key nutrients. The bacteria we share with our partners are often harmless and sometimes benefit our health rather than hindering it.

Janelle Mwerinde is doing a PhD in skin microbiology sponsored by and affiliated with the Walgreens Boots Alliance.

Conor Meehan does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.