Kissing Transfers 80 Million Bacteria Between Partners

Bacterial build-up on the human
tongue (source).

As many as 80 million bacteria are transferred during a 10 second kiss.  I'm gonna let that sink in for my fellow germophobes.


The 100 trillion microorganisms that make up the human microbiota are responsible for all kinds of important process, from digestion to synthesis to protection against disease.  Microbiota composition is dependent on a bunch of factors, including age, diet, genetics, the environment, and who we interact with.  Your mouth contains 700 varieties of bacteria, and these are influenced by the people you keep close to you, according to a new open access study published in Microbiome yesterday.

Study design

The researchers, led by Remco Kort from the Netherlands Organisation for Applied Scientific Research (TNO), studied 21 coupled between the ages of 17 and 45.  They sampled the tongue surface and saliva of both partners before and after a 10 second kiss.  One of the partners then took a probiotic yogurt drink, and after another 10 second kiss, saliva was collected from the other partner.  The 16S rRNA of the samples were sequenced to identify which bacterial species were present in the mouths and saliva of each partner.  The researchers also looked at the amount of bacteria that was transferred from one partner to another.

The results basically showed that intimate partners have more microbial communities in common than strangers do.  And all it takes is 9 intimate kisses per day to do it.  An intimate kiss doesn't increase the amount of bacteria in your mouth, though, likely because both partners are trading similar amounts of bacteria.  So while you're receiving 80 million bacteria from your partner, you're also passing on approximately the same amount.

Intimate kissing is involved in the romantic courtship rituals of 90% of human cultures.  It's also unique in the animal kingdom to humans.  Based on their results, the authors posit that kissing may have an evolutionary significance: kissing maintains a consistent oral microbiota between partners, as do other factors like a shared lifestyle and diet.  They can't speculate any further than that, but based on the importance of the human microbiome, it stands to reason that sharing the same bacterial species as partners, friends, and children was evolutionarily crucial.