You swap more than spit.

You are more than just your own cells. Your body is a vast ecosystem populated by bacteria. These trillions of bacteria that call you home — collectively called the “microbiome” — live in your gut, on your skin, in your organs, everywhere. And as we’ve started studying our single-celled tenets, we’ve linked changes in their daily lives to everything from diabetes to obesity to cancer. These bacteria outnumber our own cells by maybe an order of magnitude, so of course changes in their lives lead to changes in ours. Changing sleep patterns or food intake or even taking antibiotics can rapidly and drastically change our microbiomes, but new research says that kissing can too. An intimate kiss just ten seconds long can infuse a partner’s mouth microbiome with 80 million bacteria.

A new study published today in the journal Microbiome has found that frequent intimate kissing can change the make-up of microbes in your mouth. Researchers in the Netherlands sampled the mouths of 21 couples for their microbial communities, as well as before and after intimate kissing (after eating a probiotic yogurt that the researchers could track). They also asked the couples about kissing behavior and kissing frequency over the last year. (And of course, 74% of the men reported higher intimate kiss frequencies than the women of the same couple.)

The team — lead by Remco Kort, Professor of Microbial Genomics at Amsterdam’s VU University — found that members of the same couple had more similar tongue microbiota than unrelated individuals, but not more similar bacteria in the saliva. Additionally, after kissing the couples did not share more similar bacteria on the tongue or in the saliva. This suggests that the tongue is a more permanent home for bacteria, and while it doesn’t change after a single kiss, it will change over time as couples live together, kiss frequently, and share food, for example. And the fact that saliva’s bacterial tenets aren’t more similar in couples suggests that the saliva in the mouth might be too transitory to establish a trend.

Lactobacillus: These rod-shaped bacteria were used in the probiotic yogurt to check the transfer of microbes in couples. Sexy.

But though saliva comes and goes, the study also found that its bacteria does get more similar in a couple that reports a higher kiss frequency. Specifically, the bacteria in couples’ saliva is most similar up until one and a half hours after kissing and in couples that report having more than nine kisses per day.

Clearly, microbes are going to hitch a ride when you have an intimate kiss with someone, but how many make it over? To find that out the researchers conducted a “controlled kissing experiment” (very romantic) where they had a partner down some probiotic yogurt containing microbes with known genetic codes. After the intimate kiss, the team looked for copies of these codes in the “receiver’s” mouth and (making a few assumptions) calculated that around 80,000,000 bacteria could be transferred by a kiss lasting just ten seconds.

It all sounds pretty gross, I know, but understanding the transfer of bacteria in, on, and around our bodies has so far proven incredibly important. For example, when a baby is born a mother’s microbiome is imprinted onto the newborn, which then goes on to (most likely) influence the baby’s future health and nutrition. For this study, understanding how our mouth’s microbiomes change when we kiss could at least partially explain why humans kiss in the first place — from assessing the viability of a mate from chemical ques in the saliva to unknowingly sharing our microbiomes with those we love.

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STUDY: Shaping the oral microbiota through intimate kissing
Remco Kort, Martien Caspers, Astrid van de Graaf, Wim van Egmond, Bart Keijser and Guus Roeselers
Microbiome 2014, 2:41

IMAGE: Dirty Kiss by Athena’s Pix; Lactobacillus sp 01 by CDC