Couples Transfer 80 Million Bacteria with a Single Kiss

What happens when you kiss someone? You're transferring a lot of bacteria. Scientists have found that as many as 80 million bacteria are transferred during a 10 second kiss. Not only that, but partners that kiss each other regularly actually share the same communities of oral bacteria.

Bacteria are everywhere within our bodies. In fact, an ecosystem of more than 100 trillion microorganisms live within our bodies, called the microbiome. This is essentially for the digestion of food, synthesizing nutrients and preventing disease. This particular microbiome is shaped by genetics, diet and age, but also by the individuals with whom we interact.

In order to see if kissing could also shape bacterial communities, the scientists studied 21 couples. They asked them to fill out questionnaires on their kissing behavior, including their average intimate kiss frequency. Then, they examined their oral microbiota on the tongue and in their saliva.

"Intimate kissing involving full tongue contact and saliva exchange appears to be a courtship behavior unique to humans and is common in over 90 percent of known cultures," said Remco Kort, the lead author of the new study, in a news release. "Interestingly, the current explanations for the function of intimate kissing in humans include and important role for the microbiota present in the oral cavity, although to our knowledge, the exact effects of intimate kissing on the oral microbiota have never been studied. We wanted to find out the extent to which partners share their oral microbiota, and it turns out, the more a couple kiss, the more similar they are."

In this case, a member of each of the couples had a probiotic drink with specific varieties of bacteria. After a kiss, the researchers found that the quantity of probiotic bacteria in the receiver's saliva rose threefold. In fact, about 80 million bacteria were transferred during a 10 second kiss.

The findings reveal a bit more about why kissing may occur in the first place as a courting behavior. It could be to cause populations of bacteria to become more similar between partners.

The findings are published in the journal Microbiome.