A Ten-Second Kiss Transfers 80 Million Bacteria, Research Shows

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A researcher set out to determine whether cognitive training can help men and women better identify sexual cues. Getty

There’s very little romance in a collection of 80 million teeming bacteria. But that, in addition to affection, is what’s being exchanged in a 10-second kiss, according to a new study published Monday in the journal Microbiome.

Researchers at the Netherlands Organisation for Applied Scientific Research asked 21 couples to fill out questionnaires about their kissing habits before participating in a “controlled kissing experiment.”

The researchers collected samples from participants' saliva and tongue surfaces before asking them to share an “intimate kiss,” which they defined as involving “full tongue contact and saliva exchange.” Researchers then asked one partner to consume 50 milliliters of a probiotic yogurt drink to introduce marker bacteria they could use to measure bacterial transfer between partners. They collected samples from the partner who consumed the yogurt drink before a second timed kiss of 10 seconds, and then did the same for the other partner after the kiss.

“We identified the probiotic Lactobacillus and Bifidobacterium marker bacteria in most kiss receivers, corresponding to an average total bacterial transfer of 80 million bacteria per intimate kiss of 10 [seconds],” the researchers wrote.

While one kiss did not make their oral microbiota significantly more similar, the researchers found a clear correlation between couples’ kissing frequency and the level of similarity between their salivary microbiota. The same applied to the amount of time passed since their last kiss. Microbiota refers to the composition of microorganisms that inhabit a certain part of the body. The shared salivary microbiota was most pronounced in couples that kissed at least nine times per day, or who were tested less than an hour and a half since their last kiss.

In short, the more frequent a couple’s kisses, the more similar their salivary microbiota was likely to be. That’s romance.

The study also showed that on average, the oral microbial communities of partners—especially those on the surface of the tongue—are more similar than those of two random individuals. However, the similarity in tongue surface bacteria was not correlated with kissing habits, suggesting that shared lifestyle, environment or other factors are at play.

Researchers found that some of the bacteria in common in couples was more transient than others.

"French kissing is a great example of exposure to a gigantic number of bacteria in a short time," the study’s lead researcher Remco Kort told BBC News.

"But only some bacteria transferred from a kiss seemed to take hold on the tongue. Further research should look at the properties of the bacteria and the tongue that contribute to this sticking power."

According to BBC News, the researchers collaborated with the Micropia museum in Amsterdam, where a Kiss-o-meter exhibition can instantly tell visitors how many microbes they’ve exchanged in a kiss—just in case you needed an idea for your next date.