According to a study published in the open access journal Microbiome, a 10-second kiss will result in the transfer of up to 80 million bacteria. The study also found that kissing each other more than nine times a day between partners would result in similarities in their oral microbiota. The human system has more than 100 trillion microorganisms, or microbiota. They are essential for food digestion, nutrient synthesis, and disease prevention. They are influenced by genetics, diet, age, and the people we come in contact with. There are over 700 different species of bacteria in the mouth, and their oral microbiota is often influenced by the people we are close to. Researchers from the Netherlands Organization for Applied Sciences (TNO) and the Microbiology Collection (Micropia) surveyed 21 couples through a questionnaire on kissing behavior, including kissing frequency. Swabs of the tongue and saliva were then collected from the subjects to analyze the microbial composition of each couple’s mouth. The results showed that a high frequency of intimate kissing between couples would lead to a significant similarity in their salivary microbiota. An average of more than 9 intimate kisses per day resulted in a significant similarity in the oral microbiota of the partners. Remco Kort Remco Kort, lead author of the study, is from the National Organization for Applied Scientific Research and the Department of Systems Biology in the Netherlands and is a consultant to the Micropia Museum of Microbiology. He said, “Intimate kissing involving tongue contact and saliva exchange is common in more than 90% of known cultures and appears to be a uniquely human courtship behavior. Interestingly, the main explanation for the role of this intimate kissing is that it is important for microbes in the mouth, while the exact role of intimate kissing for oral microbes has not yet been studied. We wanted to find out to what extent partners share their oral biota. The results showed that the more frequently couples kissed intimately, the more similar the oral microbiota was. In a bacterial transfer-controlled kissing experiment, one partner in the couple drank a drink containing probiotics that contained specific lactobacilli and bifidobacteria, among others. After the kiss, the researchers found that the number of probiotics in the saliva of the non-drinking partner was three times higher, and it was estimated that more than 80 million oral microbiota were transferred during a 10-second kiss. The study also suggests that other selection mechanisms have an important role in oral microbial composition. Shared lifestyle, diet, and personal hygiene habits in particular have an important impact on the biota on the tongue. The researchers also found a higher similarity of microbiota on the tongues of partners than unrelated individuals, but this similarity did not increase with kissing frequency, in contrast to salivary biota. Among the results of the questionnaire, the researchers found an interesting phenomenon in that 74% of men reported couples kissing intimately more frequently than women, with men reporting an average of 10 kisses per day, while women reported an average of 5. Men reported fully twice as much as women. The authors relied on calculations using average transfer and some assumptions about bacterial transfer related to kissing contact area and average saliva volume to calculate the amount of bacterial transfer in kissing.