A 10-second kiss transfers as many as 80 million bacteria between partners.
And, according to research published in the journal Microbiome, those who press lips at least nine times a day share similar communities of oral bacteria.
The ecosystem of more than 100 trillion microorganisms in our bodies – the microbiome – is essential for the digestion of food, synthesising nutrients and preventing disease.
It is shaped by genetics, diet, and age but also the individuals with whom we interact. With the mouth playing host to more than 700 varieties of bacteria, the oral microbiota also appear to be influenced by those closest to us.
Researchers from Micropia and TNO in the Netherlands studied 21 couples, asking them to fill out questionnaires on their kissing behaviour including their average intimate kiss frequency.
They then took swab samples to investigate the composition of their oral microbiota on the tongue and in their saliva.
The results showed that when couples intimately kiss at relatively high frequencies, their salivary microbiota become similar. On average it was found that at least nine intimate kisses per day led to couples having significantly shared salivary microbiota.
Lead author Remco Kort, from TNO’s Microbiology and Systems Biology department and adviser to the Micropia museum of microbes, says intimate kissing involving full tongue contact and saliva exchange appears to be a courtship behaviour unique to humans and is common in 90% of known cultures.
An interesting but separate finding of the study was that 74% of the men reported higher intimate kiss frequencies than the women of the same couple.
This resulted in a reported average of 10 kisses per day from the males, twice that of the female reported average of five per day.
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