Mouth to mouth: Kissing transfers 80 million bacteria, scientists say

Goodbye kiss
A 10-second “intimate kiss” can transfer 80 million bacteria from mouth to mouth, according to a new study.
(Fredrik von Erichsen / European Pressphoto Agency)

Scientists have demonstrated that boys (well, men) really do have cooties (a simplified term for bacteria) that can spread to girls (OK, women) by kissing them.

In fact, a 10-second “intimate kiss” can transfer a whopping 80,000,000 bacteria from one mouth to another, according to a new report in the journal Microbiome.




An earlier version of this post said one volunteer’s claim of kissing his partner 50 times a day would add up to 500 minutes of kissing. It would be 500 seconds, or 8 minutes and 20 seconds.


A team of Dutch researchers recruited 21 couples who happened to be visiting the Artis Royal Zoo in Amsterdam on a summer day. All 42 volunteers (whose ages ranged from 17 to 45) allowed the researchers to wipe their tongues with a cotton swab several times. They also agreed to spit into sterile tubes and answer questions about their kissing habits.

The researchers found that the particular community of bacteria living on a volunteer’s tongue was more similar to the bacteria on his or her kissing partner’s tongue than to a stranger’s tongue. They quantified this using a measure called the Morisita-Horn index, where 0 indicates complete overlap and 1 means no overlap at all. The MH index value for kissing couples was 0.37, significantly lower than the 0.55 for strangers.


Then the volunteers engaged in some public displays of affection (a 10-second kiss “involving full tongue contact and saliva exchange”) and had their tongues swabbed again. According to the bacterial analysis, a fresh kiss barely budged the similarity index value. That suggests that the overlap in tongue bacteria is probably “a long-term effect of couples living together” – sharing meals, toothpaste and other items from daily life.

In a further test, some of the volunteers were given a probiotic yogurt drink spiked with a marker bacteria. Researchers swabbed their tongues and asked them to kiss their partners. Then the partners had their tongues swabbed. Comparing the contents of the yogurt-drinkers’ swabs and their partners’ swabs, the researchers calculated that a single kiss can deposit 80,000,000 bacteria from one tongue to another.

A single kiss had no immediate effect on the composition of bacteria in the saliva samples, the researchers found. But swapping spit certainly entails swapping bacteria as well. The more often a couple kissed, the more similar the bacteria in their saliva samples. The scientists calculated that couples had to kiss at least nine times a day to achieve an MH value below 0.5.

And here’s a bonus finding: The men said they kissed their partners an average of 10 times a day, while the women recalled kissing only five times a day. The researchers concluded that the men were most likely exaggerating the frequency of their romantic encounters. Most egregiously, one male volunteer said he averaged 50 intimate kisses per day. The researchers declared this estimate “unrealistically high.” Besides, his partner said they kissed only eight times per day.


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