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Just a single kiss can transfer 80 million bacteria, a new study reported. According to the team of Dutch scientists, couples who kissed for 10-seconds nine times a day were more likely to share mouth bacteria with each other than couples who did not kiss as often.

"Intimate kissing involving full tongue contact and saliva exchange appears to be a courtship behavior unique to humans and is common in over 90% of known cultures. Interestingly, the current explanations for the function of intimate kissing in humans include an 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," the lead investigator of the study, Professor Remco Kort, said in the press release. "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."

For this study, the team from the Netherlands Organization for Applied Scientific Research (TNO) recruited 21 couples and asked them questions regarding their kissing habits. The couples then kissed for 10-seconds, which the team strictly timed. The researchers collected samples before and after the kiss from the participants' tongues and saliva.

Before the second kiss, one partner had to drink a probiotic drink that contained an easily identifiable combination of bacteria. After the 10-second kiss, the team assessed the bacteria in both participants again.

The researchers discovered that after a kiss, an average of 80 million bacteria were transferred. They added that the combination of bugs found in saliva were more likely to be different after a kiss whereas the bug populations present on the tongue tended to remain the same.

"French kissing is a great example of exposure to a gigantic number of bacteria in a short time," Kort, from TNO's Microbiology and Systems Biology department and adviser to the Micropia museum of microbes, said according to 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. These types of investigations may help us design future bacterial therapies and help people with troublesome bacterial problems."

The study, "Shaping the oral microbiota through intimate kissing," was published in the journal, Microbiome.