Australasian Science: Australia's authority on science since 1938

The Unromantic Truth About Kissing


Credit: purmar/iStockphoto

By Magdeline Lum

When couples kiss intimately for 10 seconds they transfer 80 million bacteria.

Up to 80 million bacteria are transferred during a 10-second kiss, and couples that kiss up to nine times a day have similar communities of oral bacteria according to research published in Microbiome.

This number pales in comparison to the 100 trillion microbes that live in our bodies and are essential for digesting food and preventing disease. The human mouth is home to around 700 varieties of the bacteria living in the human body.

The study by researchers from the Micropia museum of microbes in Amsterdam and the Netherlands Organisation for Applied Scientific Research (TNO) quizzed 21 couples about their kissing behaviour, including their average intimate kiss frequency, and also collected swab samples to investigate the bacteria living on their tongues and in their saliva. For the purposes of the study, an intimate kiss was defined as “involving full tongue contact and saliva exchange”.

The results showed that when couples kiss at relatively high frequencies their salivary bacteria became similar. On average it was found that at least nine intimate kisses per day gave couples significantly shared salivary microbiota.

“Intimate kissing involving full tongue contact and saliva exchange appears to be a courtship behaviour unique to humans and is common in over 90% of known cultures,” said lead author Remco Kort from TNO’s Microbiology and Systems Biology Department. “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. 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.”

The 21 couples, aged 17 to 45, were also involved in a controlled kissing environment to enable the researchers to quantify how much oral bacteria was exchanged. One member of each couple drank a probiotic formulation of specific varieties of bacteria, including Lactobacillus and Bifidobacteria. After an intimate kiss, oral swab samples showed that the number of probiotic bacteria in the receiver’s saliva rose threefold. From this the researchers calculated that up to 80 million bacteria were transferred during a 10-second kiss.

What happens to the bacteria after the kiss is still a matter of debate. Researchers surmise that due to the high flow rate of saliva and low content of nutrients in it, bacteria would struggle to proliferate. The bacteria in saliva were also likely to be dislodged from oral surfaces like the tongue. The researchers therefore concluded that an almost constant bacterial exchange is needed to maintain a shared salivary microbiota.

The study also suggests an important role for other mechanisms that select oral microbiota, such as a shared lifestyle, dietary and personal care habits, and this is especially the case for microbiota on the tongue. The researchers found that while tongue microbiota were more similar among partners than unrelated individuals, their similarity did not change with more frequent kissing, in contrast to the findings on the saliva microbiota.

The kissing questionnaire revealed that 74% of the men reported higher frequencies of intimate kissing than their partner. This resulted in a reported average of ten kisses per day from the males, while the females reported an average of five per day.