French kiss, tongue swapping, tonsil hockey: whatever it’s called, people like to kiss.

And while a 10-second “intimate kiss” can transfer 80 million bacteria from one mouth to another, according to a new report in the journal Microbiome, that’s nothing compared to the trillions of bacteria we all carry.

A team of Dutch researchers recruited 21 couples who happened to be visiting the Artis Royal Zoo in Amsterdam on a summer day. All 42 volunteers (whose ages ranged from 17 to 45) allowed the researchers to wipe their tongues with a cotton swab several times. They also agreed to spit into sterile tubes and answer questions about their kissing habits.

The researchers found that the particular community of bacteria living on a volunteer’s tongue was more similar to the bacteria on his or her kissing partner’s tongue than to a stranger’s tongue. They quantified this using a measure called the Morisita-Horn index, where 0 indicates complete overlap and 1 means no overlap at all. The MH index value for kissing couples was 0.37, significantly lower than the 0.55 for strangers.

Then the volunteers engaged in some public displays of affection (a 10-second kiss “involving full tongue contact and saliva exchange”) and had their tongues swabbed again. According to the bacterial analysis, a fresh kiss barely budged the similarity index value. That suggests that the overlap in tongue bacteria is probably “a long-term effect of couples living together” – sharing meals, toothpaste and other items from daily life.

In a further test, some of the volunteers were given a probiotic yogurt drink spiked with a marker bacteria. Researchers swabbed their tongues and asked them to kiss their partners. Then the partners had their tongues swabbed. Comparing the contents of the yogurt-drinkers’ swabs and their partners’ swabs, the researchers calculated that a single kiss can deposit 80,000,000 bacteria from one tongue to another.

Shaping the oral microbiota through intimate kissing

Microbiome 2014; 2:41

Remco Kort, Martien Caspers, Astrid van de Graaf, Wim van Egmond, Bart Keijser and Guus Roeselers

http://www.microbiomejournal.com/content/2/1/41

Abstract

Background

The variation of microbial communities associated with the human body can be the cause of many factors, including the human genetic makeup, diet, age, surroundings, and sexual behavior. In this study, we investigated the effects of intimate kissing on the oral microbiota of 21 couples by self-administered questionnaires about their past kissing behavior and by the evaluation of tongue and salivary microbiota samples in a controlled kissing experiment. In addition, we quantified the number of bacteria exchanged during intimate kissing by the use of marker bacteria introduced through the intake of a probiotic yoghurt drink by one of the partners prior to a second intimate kiss.

Results

Similarity indices of microbial communities show that average partners have a more similar oral microbiota composition compared to unrelated individuals, with by far most pronounced similarity for communities associated with the tongue surface. An intimate kiss did not lead to a significant additional increase of the average similarity of the oral microbiota between partners. However, clear correlations were observed between the similarity indices of the salivary microbiota of couples and self-reported kiss frequencies, and the reported time passed after the latest kiss. In control experiments for bacterial transfer, we identified the probiotic Lactobacillus and Bifidobacterium marker bacteria in most kiss receivers, corresponding to an average total bacterial transfer of 80 million bacteria per intimate kiss of 10 s.

Conclusions

This study indicates that a shared salivary microbiota requires a frequent and recent bacterial exchange and is therefore most pronounced in couples with relatively high intimate kiss frequencies. The microbiota on the dorsal surface of the tongue is more similar among partners than unrelated individuals, but its similarity does not clearly correlate to kissing behavior, suggesting an important role for specific selection mechanisms resulting from a shared lifestyle, environment, or genetic factors from the host. Furthermore, our findings imply that some of the collective bacteria among partners are only transiently present, while others have found a true niche on the tongue’s surface allowing long-term colonization.