A single kiss delivers 80 million bacteria
While that's a striking figure, you might also wonder how they counted.
In "a controlled kissing experiment", one member of each of the couples taking part had a probiotic drink containing specific varieties of bacteria including Lactobacillus and Bifidobacteria.
After an intimate kiss – defined, unromantically, as "involving full tongue contact and saliva exchange" – the researchers found that the probiotic bacteria in the receiver's saliva rose threefold. They then calculated that in total 80 million bacteria would have been transferred.
It sounds a lot, but is only a tiny percentage of the ecosystem of more than 100 trillion microorganisms that live in our bodies - the microbiome.
The mouth is host to more than 700 varieties of bacteria, the oral microbiota also appear to be influenced by those closest to us.
In fact, the kissing research, carried out by independent research organisation TNO and Micropia, an Amsterdam museum dedicated to microbes and micro-organisms, found that partners who kiss each other at least nine times a day share similar communities of oral bacteria.
"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," said lead author Remco Kort, from TNO's Microbiology and Systems Biology department and adviser to the Micropia museum of microbes.