What’s in a kiss? About 80 million bacteria

When you share a kiss, you’re sharing affection. But according to new research published in Microbiome, you’re also sharing up to 80 million bacteria.

A smooch for science

Researchers aimed to study the relationship kissing has on the human microbiome—an ecosystem of more than 100 trillion microbes, like bacteria, that live in our bodies and play an important role in human health.

The microbiome is shaped by genetics, diet, age and the people with whom we interact. And the mouth is a warm, moist hotbed for a bunch of this microbial activity. It’s home to more than 700 varieties of bacteria.

To explore the relationship between lip locking and bacterial exchange, researchers studied the kissing behavior of 21 couples who were visiting a zoo in Amsterdam.

Participants completed questionnaires about their kissing behavior. Then, researchers took swab samples to analyze the bacteria on people’s tongues and in their saliva.

Researchers also asked the couples to kiss in a controlled experiment to measure the transfer of bacteria. One member of the couple drank a probiotic beverage that contained health-promoting bacteria. Then, the couple shared an 10-second intimate kiss—with full tongue contact and saliva exchange. Researchers took multiple samples to see if certain bacteria from the drink transferred between the partners during the kiss.

The researchers compared the similarities of the samples between partners and between unrelated participants. They also studied the differences between a person’s own samples.

They found that while an intimate kiss can swap up to 80 million bacteria, kissing may not have that much of an effect on a couple’s shared microbes, unless they kiss intimately at least 9 times a day. And although the microbes on the tongue were more similar in couples than in unrelated participants, the researchers suggested that this similarity could be less related to kissing and result instead from living together, having the same diet or sharing personal-care habits.

What to make of mouthy microbes

So is the bacterial exchange in kissing a bad thing? Not necessarily, according to the American Academy of Microbiology. The human body has up to 3 times more bacterial cells than human cells, and they are essential for things like:

  • Digesting food.
  • Synthesizing nutrients.
  • Preventing disease.

Researchers are still investigating exactly what makes up a healthy microbiome. But these are some things to think about next time you share a smooch. To learn more, read the full study in the journal Microbiome.


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