Your gut is home to around 40 trillion microbes. In return for the living space you offer, they perform key functions that only they can do.
Humans and microbes have evolved together over time – it’s a two-way interaction and a long-term partnership.
In a similar way, your gut microbiome evolves based on your two-way interactions with others and your long-term partnerships.
In this article, we’ll dive into how your gut microbiome is shaped by those around you. Spoiler alert – it’s a lot more than you might think…
1. You share microbes with the people you live with…
True: You are constantly swapping microbes with other people, but you share the most microbes with the people who you spend the most time with. Long-term couples tend to have more similar gut microbiomes.
Microbes can be exchanged through skin contact, the air, and shared surfaces, meaning that sitting next to someone, hugging, and kissing all allow this transfer.
And closeness matters too. Couples reporting a ‘close’ relationship had increased gut microbiome diversity and richness compared with people reporting ‘somewhat close’ relationship.
But exactly how many microbes are shared? It’s been estimated that you share around 12% of your gut microbes with people you live with.
2. …and the longer you live together, the more microbes you’re likely to share
True: Over time, long-term couples often develop more similar gut microbiomes due to shared lifestyles and daily habits.
For example, diet plays a fundamental role in shaping the composition and function of the gut microbiome. Because long-term partners tend to eat similar foods and share meals, this further promotes microbial exchange and overlapping similarities in their gut microbiomes.
In fact, the time since you started living with your long-term partner influences your gut microbiome more than age or genetics.
3. Feelings of love are not influenced by microbes
False: Growing science suggests the gut microbes can regulate hormones that drive attraction and attachment – like serotonin, dopamine and oxytocin - and potentially shape emotional states, including love.
For example, even though serotonin is mostly linked to the brain - about 95% - is produced by cells lining the gut. Gut-based serotonin cannot pass into the brain, but it can still influence brain serotonin levels indirectly – by stimulating the vagus nerve (the main nerve connecting the gut and the brain) which sends signals upward.
What’s more, many microbes can directly produce serotonin themselves. For example, bacteria in the Enterobacteriaceae family. Microbes can indirectly influence serotonin levels through the production of SCFAs.
4. A single kiss transfers fewer than a hundred microbes
False: The mouth harbours the second largest and most diverse microbiome after the gut with microbes colonising the teeth and gums. Saliva, with a near-neutral pH 6.5-7, provides a favourable environment for most species of bacteria.
Like the gut microbiome, studies show that couples have similarities in their oral microbiomes too. in fact, studies have shown that you share around 32% of your oral microbes with your partner. But it doesn’t relate to how often couples kiss, suggesting that shared lifestyles and living spaces might play a bigger role.
What’s more, a 10 second kiss shares 80 million bacteria!
Summary
In summary, microbes are constantly exchanged. Your gut microbiome isn’t shaped only by your own habits - those around you play a key role as well. Over time, your gut microbiome begins to resemble that of the person you live with.
Sources
Close social relationships correlate with human gut microbiota composition. Scientific Reports. (2019). Link.
The person-to-person transmission landscape of the gut and oral microbiomes. Nature. (2023). Link.
Does a microbial-endocrine interplay shape love-associated emotions in humans? A hypothesis. mSystems. (2025). Link.
The multifaceted role of serotonin in intestinal homeostasis. International Journal of Molecular Sciences. (2021). Link.
Oral microbiome: Unveiling the fundamentals. Journal of Oral and Maxillofacial Pathology. (2019). Link.
Shaping the oral microbiota through intimate kissing. Microbiome. (2014). Link.
