One kiss 'shares 80 million bugs'
A single 10-second kiss can transfer as many as 80 million bacteria, according to Dutch scientists.
They monitored the kissing behaviour of 21 couples and found those who kissed nine times a day were most likely to share salivary bugs.
Studies suggest the mouth is home to more than 700 different types of bacteria - but the report reveals some are exchanged more easily than others.
The research is published in the journal Microbiome.
A team from the Netherlands Organisation for Applied Scientific Research (TNO) asked 21 couples a series of questions to assess their kissing habits, including how frequently they had kissed in the last year and when they last locked lips.
Scientists took bacterial samples from the volunteers' tongues and saliva before and after a strictly timed 10-second kiss.
One member of the couple then drank a probiotic drink, containing an easily identifiable mixture of bugs.
On the couple's second kiss, scientists were able to detect the volume of bacteria transferred to the other partner - on average 80 million bacteria in a single 10-second kiss.
But while bacteria in the saliva seemed to change quickly in response to a kiss, bug populations on the tongue remained more stable.
Prof Remco Kort, who led the research, said: "French kissing is a great example of exposure to a gigantic number of bacteria in a short time.
"But only some bacteria transferred from a kiss seemed to take hold on the tongue.
"Further research should look at the properties of the bacteria and the tongue that contribute to this sticking power.
"These types of investigations may help us design future bacterial therapies and help people with troublesome bacterial problems."
The Dutch scientists worked in collaboration with the museum Micropia, the world's first museum of microbes, based in Amsterdam.
In a newly opened exhibition, couples are invited to share a kiss and are provided with an instant analysis of the bugs they have exchanged.
A growing number of researchers are looking at the microbiome - an ecosystem of some 100 trillion micro-organisms that live in and on our bodies.
Scientists say these populations may be essential for health and the prevention of disease.