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Oral Microbiota: The Unforeseen Love Doctors Involved in “Intimate Kissing”

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By: burgundy bug

Two tongues licking the same pink, gold heart covered lollipop

Source: Adobe Stock

You don’t just share an intimate moment during a kiss; you also share millions of microbiota.

That’s right. As you lock eyes and greet each other with your slimy, beastly tongues, microscopic bacteria thrash about from your mouth to theirs for some mingling — and vice versa.

Kissing isn’t any reinvention of the wheel, either. According to a 2015 Microbiome Journal study, mouth-to-mouth contact is commonplace among other species, including fish, birds, and primates.

However, making out — or as the scientific community calls it, “intimate kissing” — does present itself as a twist to kissing that’s unique to humans.

Why Do We Kiss?

Although the current explanations for the role of intimate kissing involve oral microbiota and viruses, little research has been done on how intimate kissing affects these organisms.

“Kissing may contribute in mate assessment and bonding via sampling of chemical taste cues in saliva, including those resulting from the metabolic activity of the bacterial community on the surface of the tongue,” the study says.

A.K.A. — the reason we kiss may function as a way to “taste test” or “sample” the bacteria colonizing on the tongue of our partner (or potential mate). Ewww.

It’s also theorized that kissing may have evolved as a form of immunization. A Medical Hypotheses journal article suggests kissing pre-emptively protects pregnant women by exposing them to human cytomegalovirus.

This virus travels readily through bodily fluids such as saliva, urine, and semen. But contracting the virus prior to pregnancy would cause far less severe symptoms.

Speaking of Which…

An editorial article in the Journal of Laboratory Physicians ponders the possibility of “customizing” the establishment of an infant’s microbiome immediately after their birth.

The idea is founded upon an Indian tradition that posits a newborn isn’t to consume any breastmilk or fluids until it has sucked the honey off of the index finger of a highly-respected and “legendary” woman chosen by the family. It’s believed that the child will be blessed and grow to have the same personality and health as the chosen woman.

But what if it’s more than just a tradition? Perhaps the “legendary woman” is passing on her “legendary microbes” from the tip of her index finger to the baby.

“We know that fingers are the host of various personal and environmental microbiota, which can be divided into residual and transient types,” the editorial piece explains. “The count of culturable bacteria can be several thousands on the fingertips, and especially in ancient times, where sanitary toilets were not available, the hands were used to clean the anus after defecation, the fingers carried a plethora of gut microbes.”

… Well, that’s a lot to digest.

Read: Think With Your Gut

The Burgundy Zine

But maybe in addition to “designer babies,” we’ll see modern science make an industry out of “designer oral and gut microbiomes for newborns.”

Okay, Back to Intimate Kissing

“However, both functions for intimate kissing, mate assessment or some form of immunization, involve an important role for the viruses and microorganisms that reside in our mouth,” the Microbiome journal study resumes.

Just so you know, there are two main types of surfaces in your mouth that house microorganisms: non-shedding surfaces, which are essentially just your teeth, and shedding surfaces, which encompass your tongue, hard palate, soft palate, cheeks, and lips.

Although there’s a lot of bacteria in your saliva, it doesn’t provide very hospital conditions for them: saliva has a high flow rate and lacks nutrients.

Instead, the bacteria in your saliva are more or less those shed or dislodged from oral surfaces, particularly the top surface of your tongue.

Cropped photo of a woman smiling with her tongue out

Source: Adobe Stock

How Kissing Affects Your Oral Microbiota

In the Microbiome journal study, 21 couples had their oral microbiota assessed before and after kissing. Then, they ate a probiotic yogurt and had their oral microbiota assessed once more after kissing again.

The researchers discovered that couples share some of the same microbiota from the surfaces of their tongues “for at least hours after kissing.” This suggests “collective bacteria” have taken hold to colonize the recipient’s tongue.

Some of these bacteria only colonize briefly, while others take up a more permanent residency.

Additionally, the researchers found the microbial composition was far more similar between couples than those who are unrelated to them.

These similarities could result from a long-term effect of the couples living with one another. After all, cohabitating would cause some overlap in dietary and personal care habits.

This explanation also fits in well with the previous finding that those who live in the same household (couples, in particular) have more microbiota in common with each other than they do with those in other households.

Additionally, the more often couples said they kissed, the more similar their salivary microbial compositions were. Yet, kiss frequency didn’t appear to correlate with tongue microbiota similarities.

“After a single intimate kiss, we did not observe a significant effect on the similarity of the salivary and tongue microbiota,” the study adds.

Thus, their data indicates a “large number of kisses (at least nine per day)” are required to cause significantly similar salivary microbiota.

“The effect of a single kiss is limited.”


Taking the sample within an hour and 45 minutes increased the overlap in salivary microbiota between couples, as well.

Yogurt Kisses

Fresh yogurt with raspberries, served in miniature glass jars with berry-pink yogurt oozing down the sides

Source: Envato Elements

To gauge how couples were transmitting their bacteria, researchers had the couples slurp up a probiotic yogurt drink containing Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, which are common yogurt bacteria.

The beverage also contained the probiotic bacteria L. rhamnosus GG, L. acidophilus LA5, and B. lactis BB12.

Then, the researchers measured how present these bacteria are in the oral cavity vs. the yogurt drink.

Since Lactobacillus and Bfidobacteria were the least present in the oral cavity (on average, these constitute 0.15 percent of salivary bacteria and 0.01 tongue bacteria), they decided to use these types of microbiota to measure how many were present after drinking the yogurt and how many were present after just kissing someone who drank the yogurt.

After licking their lips clean of the yogurt drink, levels of Lactobacillus and Bfidobacteria increased to roughly eight and 13 percent in the participants’ mouths, respectively.

Whereas levels of these bacteria increased to 0.54 and 0.49 percent in those who received a yogurt kiss — that’s a significant uptick from 0.01 to 0.15 percent, wouldn’t you say?

In Conclusion

Kissing your partner at least nine times per day (which is considered a “large number of kisses”), or having your mouth swabbed within 90 minutes of your last kiss, results in substantially similar levels of oral bacteria.

If you are what you eat — quite literally due to how diet affects your gut microbiome — then perhaps you are who you kiss.

… And maybe that’s something you should keep in mind this Valentine’s Day.

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burgundy bug


A cynical optimist and mad scientist undercover, burgundy bug is the editor, graphic designer, webmaster, social media manager, and primary photographer for The Burgundy Zine. Entangled in a web of curiosity, burgundy bug’s work embodies a wide variety of topics including: neuroscience, psychology, ecology, biology, cannabis, reviews, fashion, entertainment, and politics. You can learn more about working with burgundy bug by visiting her portfolio website: burgundybug.com

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