Your mouth is brimming with microorganisms. Trillions of different types of bacteria have evolved to live in every little nook and cranny. They live between your teeth and set up shop among the bumps on your tongue.
While this may sound a little gross, that’s because we’ve been raised to believe that bacteria are harmful, when in fact most bacteria are just trying to help.
So when you lean in for a passionate kiss with your partner, you’re not just getting a surge of the love hormone oxytocin, you’re also exchanging about 80 million microbes. We’re not talking a peck – we mean at least a 10-second French kiss.
To find these numbers, researchers put together a study that examined 21 couples from the ages 17 to 45.  That’s right, these couples got to make out for science.
First, to establish a baseline, researchers sampled each partner’s mouth and found that these couples already had similar oral microbiomes to begin with, especially in the bacteria found on the surface of the tongue. This could be because they usually share similar habits such as oral hygiene or smoking, but it’s also likely that they regularly exchange microbes.
Compared to strangers, these couples shared similar oral microorganisms. So to be able to test the amount of bacteria swapped, researchers had the couple kiss after one partner drank a probiotic yogurt. Two types of bacteria, Lactobacillus and Bifidobacteria, which are found in high numbers in the yogurt, were the focus of the kissing measurement.
Usually, Lactobacillus and Bifidobacteria make up 0.15% of a person’s saliva and 0.01% of all the bacteria found on the tongue – so, both these strains are found in really small numbers.  But when the scientists measured the saliva and tongues of both partners after the yogurty kiss, the levels of both bacteria rose – making up almost half the total bacteria in the saliva and on the tongue. They used these numbers to estimate that about 80 million microbes are swapped during a passionate kiss.
If you’re interested in finding out exactly how many microbes you and your partner swap with each kiss you can have this tested. The next time you're in Amsterdam, head over to the Micropia museum. 
Micropia is the world’s first museum of microorganisms has a Kiss-O-Meter where it analyzes your kiss with your partner – giving you a measurement of exactly how many and what kind of microbes you just shared.
Why Do We Kiss Anyway?
For years, scientists haven’t been able to answer the question as to why we kiss in the first place. There are all sorts of half answers like, “kissing feels good” and “kissing creates surges of oxytocin for bonding.”
Considering that bacteria have been influencing our behaviors to increase their chances of survival since the beginning of time, could it be that kisses are yet another behavior ingrained in us by our microscopic counterparts?
Similar to how we didn’t understand why breast milk contained so many indigestible sugars only to find out they are the perfect food for the blooming gut microbiome – could kissing be an ancient evolutionary technique specifically designed to swap microorganisms and expand our microbial diversity?
One thing we know for sure it’s that microbial diversity is essential to our health .