In just ten seconds, one kiss can transfer up to 80 million bacteria, according to a new study published in the journal Microbiome.
Partners who kiss up to nine times per day share the same communities of oral bacteria, according to the study.
"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," says lead author Remco Kort, from the Netherlands Organization for Applied Scientific Research (TNO)'s Microbiology and Systems Biology department and adviser to the Micropia museum of microbes.
Kort and his colleagues worked with 21 couples who filled out questionnaires on their kissing behavior including their average frequency of intimate kissing.
Participants' mouths were then swabbed, giving researchers the chance to investigate the composition of the microbiota living on the tongue and in the saliva.
In a kissing experiment to quantify the transfer of microbiota, one member from each couple consumed a probiotic drink containing species such as Lactobacillus and Bifidobacteria.
Participants were then asked to kiss, after which the receiver's saliva contained three times the probiotic bacteria that it had before the kiss, and researchers calculated that 80 million bacteria would have been transferred in 10 seconds of kissing.
Calculations were performed based on average transfer values and several assumptions related to bacterial transfer, kiss contact surface and the value for average saliva volume.
A surprising separate finding revealed that 74 percent of the men in the couples reported kissing more than their female partners.
On average, the males reported 10 kisses per day whereas the women reported only five.
The study was published in open-access format in the journal Microbiome.