More than 100 trillion microbes live in our bodies, helping us digest food and stay healthy -- some 700 bacteria varieties live in our mouths alone. Now, researchers show that up to 80 million bacteria can be shared during just one French kiss, and couples who kiss each other nine times a day or more share similar oral bacteria communities. The findings were published in Microbiome this week.
Our microbiome is shaped by genetics, diet, age, and also those closest to us. "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,” says Remco Kort of TNO Microbiology and Systems Biology. “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.”
So, Kort and colleagues recruited 21 couples visiting the Artis Royal Zoo in Amsterdam to fill out questionnaires about their kissing behavior, which included kiss frequency, time passed since the latest kiss, and the composition of their last meal. The team also collected saliva samples and took tongue swabs to analyze the composition of the volunteers’ oral microbiota.
“We wanted to find out the extent to which partners share their oral microbiota,” Kort explains in a news release, “and it turns out, the more a couple kisses, the more similar they are." Couples who kiss intimately at relatively high frequencies -- at least nine French kisses per day -- exhibit significantly shared salivary microbiota on average. While tongue microbiota were more similar among partners than unrelated individuals, their similarity didn’t change with more frequent kissing.
Then, to quantify the transfer of bacteria, the team conducted a controlled kissing experiment. One member of each of the couples had a probiotic drink -- containing bacteria such as Lactobacillus and Bifidobacteria, which are common in yogurts -- and then kissed his or her partner. The quantity of probiotic bacteria in the receiver's saliva rose threefold. In order to calculate the number of bacteria passed on in a kiss, the team had to rely on average transfer values and assumptions related to bacterial transfer, the kiss contact surface, and average saliva volume. As it turns out, a total of 80 million bacteria are transferred during a 10-second kiss.
And on a separate though related note, according to the questionnaire results, 74 percent of the men reported twice as many intimate kisses than women of the same couple. All of these findings were used to help design the “Kiss-O-Meter” at Micropia, a museum of microbes in Amsterdam.