Couples exchange millions of bacteria when they kiss intimately, making their oral microbiomes look more and more alike.

What’s in a kiss?

Millions of bacteria, it turns out — more than 600 species of bacteria live in the human mouth, lining the teeth, tongue, and cheeks, though they make up only a small percentage of the 100 trillion microorganisms that live in the human body. Some of these bacteria float freely in saliva, where they can be readily spread through an intimate kiss.

Kissing is pair-bonding behavior practiced by more than 90 percent of world cultures, and for people in relationships, it represents a regular source of exposure to foreign bacteria. As far as Dutch researcher Remco Kort knew, no one had yet studied how kissing changes a couple’s oral microbiome, or the collection of bacteria that naturally live in the mouth.

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Kort recruited 21 couples who were visiting the Micropia Museum in Amsterdam to participate in his study, which was published in the journal Microbiome. The couples filled out a survey about their kissing habits and gave saliva samples before and after a 10-second kiss. Then, three couples were selected for a second experiment. One partner drank a probiotic yogurt beverage and then kissed their partner, and again researchers took spit samples from both partners before and after.

To calculate the number of kisses a couple shared each day, Kort had to average what each partner reported — which, it turns out, varied quite a bit. Among 74 percent of opposite-sex couples, the male partner reported more kisses per day than the female partner, leading to a male average of 10 kisses per day and a female average of five kisses per day. Kort thinks this is consistent with a larger trend of men overreporting and women underreporting intimate activity, which is in line with past studies on the reliability of self-reports about sex.

Couples who kissed at least nine times a day on average, or had kissed within the past hour and 45 minutes, had very similar oral microbiomes. This fell in line with a previous study showing that people who live together are more likely to have similar native microbes, particularly on the skin, than strangers.

For the three couples who tried the yogurt drink, Kort measured the transfer of bacteria by looking for traces of probiotic bacteria not normally found in high quantities in the mouth. Based on the concentration of these bacteria in the partner who had not taken the probiotic drink, his team was able to estimate that a 10-second kiss transfers about 80 million bacteria.

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“Microorganisms are in general actually beneficial, but opinion of them is totally different,” said Kort, chair of microbial genomics at the University of Amsterdam and microbiologist at the TNO research center, in an interview with Healthline. “People associate bacteria with spoilage, unhygienic conditions, and disease.”

But public perception couldn’t be further from the truth. “The oral bacteria form a protective layer and they help protect us against disease-causing microorganisms,” he continued. This layer is called a biofilm, and it clings to the solid surfaces of the mouth. Invading bacteria have to penetrate the resilient biofilm before they can reach the vulnerable tissues of the mouth.

Kort explained that bacterial transfer is actually a good thing. “What you’re doing is getting exposed to many bacteria.You gain additional species. In general, in microbiology, if you have more species you build up resistance. From this perspective, kissing is healthy.”

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