Kissing is perhaps the most direct way in which we transfer our microbiomes to one another. In fact, there is even speculation that kissing may have evolved as a way of probing a potential partner’s microbiome for compatibility! An investigation into how this microbial swap occurs was recently published in the journal Microbiome. In the article, researchers measured the oral microbiome of couples that engaged in intimate kissing.

The researchers collected salivary samples and tongue swabs from 21 partners in Amsterdam, Netherlands and asked them how often they kiss, and the time of the last kiss. They discovered that partners that kiss have more similar tongue microbiomes than those that don’t, and this similarity loosely increases with the frequency of kissing. In addition, they discovered that the salivary microbiome is quite transient, and after 2 hours there is little retention of any swapped bacteria. Finally they used yogurt to answer the age-old question: how many bugs are swapped during the average kiss? The answer: 80 million!

Remember that the next time you are kissing someone new you are actually involved in a genetic swap of 80 million bacteria. If your partner’s breath smells bad or saliva tastes bad, remember that it’s just his or her microbiome, but he or she is probably not ‘the one’.