Could your summer fling change your microbiome?

It’s summertime, when couples take off on romantic vacations and singles flock to exotic locations for adventure and perhaps a summer fling. Whatever the case, if smooching is on the itinerary, travelers may be returning home with more than a suntan and tacky souvenirs.

When we lock lips and open our mouths, we’re exchanging more than saliva; we’re altering our oral microbiota. Given that simply living with another individual can change our microbiome, and that even dog owners share microbes with their furry friends, it makes sense that kissing would have an effect, too. How much of an effect is that, you ask? Well, it depends on the length of time a couple is together and just how much kissing they do. Spoiler alert: a whole lot of bacteria are being shared.


A kiss is not just a kiss

Anyone who’s ever enjoyed a good binge of Planet Earth or a documentary about apes or monkeys knows that humans are not the only species that kiss. In fact, mouth-to-mouth contact has been observed in fish and birds, as well as our primate cousins. What sets humans apart is the French kiss; that oh-so-wonderful smooch where tongues get involved and saliva is shared. Hello you beautiful oral microbiota, you!

Even among humans, though, tongue kissing is not universal. Approximately 90 percent of cultures have some type of kissing, but it is usually platonic, such as a parent kissing a child. However, a 2015 study of 168 cultures found that only 46 percent of them engage in couple or French kissing. The researchers reported that societies with distinct social classes are usually kissers, while those with fewer social classes, like hunter-gatherer communities, are usually not. Because hunter-gathering societies can act as a window to our evolutionary past, the authors of the study hypothesized that romantic kissing is a relatively recent development in human evolutionary history.

If kissing does affect our microbiome, it’s been doing so for a long, long time. The practice of intimate kissing was firmly entrenched by the time we built civilizations, and there are numerous allusions to romantic kissing in ancient literature. The Romans referred to a deep or passionate kiss as savolium, and the poets Ovid and Catullus both praised the smooch. It’s believed that passionate kissing declined in popularity after the fall of Rome, only to make a come-back a thousand years later with the emergence of courtly love in the Middle Ages.


Kissing is more than foreplay

Regardless of how long humans have been French kissing, snogging, making out, or whatever it is you want to call it, the bottom line is that there’s nothing like a good long kiss to get any romantic encounter kicked off.  It turns out, however, that the purpose of kissing may not be to initiate sex. It all comes down, of course, to our mouth microbiome.

A 2013 study concluded that a romantic kiss actually serves a biological purpose; it helps us evaluate aspects of a potential mate’s suitability or mediate feelings of attachment in a long-term relationship, rather than to stimulate arousal and sex. Earlier research found that intimate kissing may contribute to mate assessment and bonding through chemical taste cues in the saliva.

Perhaps even more interesting, a 2010 study hypothesized that French kissing evolved to protect pregnant women against human cytomegalovirus (HCMV), a common virus that can cause abnormalities in the fetus. HCMV is easily transmitted through saliva, urine and semen, and the researchers concluded that it would cause less severe symptoms when acquired prior to pregnancy. Thus, a little tongue-dancing for mom and dad would increase her chances of contracting HCMV before it was a threat, resulting in a happier, healthier baby.


A kiss transfers a lot more than saliva

In 2014, Dutch researchers conducted a study among 21 couples to assess the effect of intimate kissing on the oral microbiota. Couples filled out a questionnaire about their kissing habits, and researchers took samples of saliva and from the surface of the kissers’ tongues. The results showed that each study participant’s oral bacteria was more similar to that of their kissing partner than to that of a stranger. The couples then kissed for ten seconds using their tongues and exchanging saliva, after which researchers took another sample. After that initial kiss, the salivary and tongue microbiota of the individuals was not significantly altered. This led researchers to conclude that the similarity in couples’ oral bacteria could be stable over the long term.

The researchers then gave one member of each couple a yogurt drink with marked bacteria (Lactobacillus and Bifidobacterium) and had each couple kiss again. Another swab revealed that a staggering 80 million bacteria had been transferred from one tongue to another.

Researchers concluded that shared oral microbiota requires frequent and recent bacterial exchanges, or kisses. It’s most pronounced in couples who kiss at least nine times per day for at least ten seconds. They noted, however, that the “microbial colonization of saliva is still a matter of debate.”

Beyond helping us learn that we transfer a lot of bacteria when we kiss, the study may contribute to the development of strategies for the prevention or cure of oral infectious diseases. Mononucleosis, known as the “kissing disease”, is usually associated with adolescence, but numerous other infections and diseases can be passed through saliva. These include  cytomegalovirus (CMV) and Streptococcus, which can cause an array of infections, including gum disease and strep throat. Saliva acts as a cleansing agent, however, and is most effective in a healthy oral microbiome.

There you have it: kissing isn’t just about sex. It’s a way to test a partner’s compatibility, birth healthier babies, and swap mouth microbes. On your summer vacation, embrace the bacterial nature of kissing by giving a smooch—or perhaps by paying a visit to the kissing booth at Micropia, the first and only museum of microbes, in Amsterdam.

Curious to learn more about your oral microbiome? uBiome’s Five-Site Explorer kit lets you see what bacteria live in your mouth, gut, nose, genitals, and skin.