A kiss is a physical exchange of love, passion and...germs.
A single 10-second kiss can transfer as many as 80 million bacteria between partners, according to Dutch scientists.
The study also found that partners who kiss each other at least nine times a day share similar communities of oral bacteria.
Researchers from the Netherlands Organisation for Applied Scientific Research (TNO) studied 21 couples, asking them to fill out questionnaires on their kissing behaviour.
Couples were asked how frequently they kissed and when they last had a smooch. The researchers then then took swab samples from participant's mouths.
The results found that at least nine intimate kisses per day led to couples having significantly shared salivary microbiota.
For the second half of the experiment, couples participated in a controlled kissing experiment. Swabs were taken before and after a strictly timed 10 second kiss.
One member of each couple then drank a probiotic drink containing specific varieties of bacteria including Lactobacillus and Bifidobacteria before the 10 second kiss was repeated.
After the second kiss, scientists found an average of 80 million bacteria had been transferred from one partner to another.
"Intimate kissing involving full tongue contact and saliva exchange appears to be a courtship behavior unique to humans and is common in over 90% of known culture," Lead author Remco Kort, from TNO's Microbiology and Systems Biology department said.
"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 research is published in the open access journal Microbiome.
The idea of sharing bacteria with a long-term partner is bad enough, but just think about all the germs you're swapping when you lock lips with a stranger.
On the plus side, bacteria isn't all bad, so although the thought of exchanging bacteria through kissing is pretty gross, in reality it won't have a negative impact on your health.