Couples who give each other intimate kisses several times a day share similar communities of saliva-dwelling oral bacteria. Romantic, right? But don't panic — many of these bacteria are essential for the digestion of food, synthesizing nutrients, and preventing disease.

Mouth-to-mouth contact has been observed in many animals, but the way we do it — intimate kisses involving full tongue contact and saliva exchange — is something that's uniquely human. It's an adaptive courtship behavior that's common to over 90% of known cultures.

We've got about 700 different varieties of bacteria in our mouth, but it now appears that our partners play an important role in its makeup. The new study, published in Microbiome, is the first to take a thorough look at the effects of intimate kissing on our oral microbiota.

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The paper by Remco Kort and colleagues shows that when couples kiss at relatively high frequencies, their salivary microbiota become quite similar. Astoundingly, as many as 80 million bacteria are transferred during a 10 second kiss.

To reach this conclusion, the researchers recruited 21 couples who answered questionnaires about their kissing behavior, and by evaluating their tongue and salivary microbiota in a controlled experiment. The researchers were able to quantify the number of bacteria exchanged by the use of marker bacteria introduced through a probiotic yoghurt drink that was ingested by one of the participants just prior to a second intimate kiss.

For couples, therefore, this research suggests that the quality and makeup of oral microbiota is strongly influenced by shared lifestyle, dietary, and personal care habits.

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Interestingly, in regards to the kissing questionnaire, 74% of the men reported higher intimate kiss frequencies than the women of the same couple. This resulted in a reported average of 10 kisses per day from the males, which was twice that of the female reported average of five per day. Hmm, so either men are over-reporting, or women are under-reporting — or more likely it's a combination of both.

Check out the entire study at the journal, Microbiome: "Shaping the oral microbiota through intimate kissing."

Image: Aliaksei Smalenski/Shutterstock.