How Many Germs Are in a Kiss?

Your mouth is home to 700 different kinds of bacteria, and you may have wondered just how many germs you’re getting—or giving!—when you swap spit with someone else. A new study, just published in the journal Microbiome, provides the answer (germaphobes, you might want to look away now): In a 10 second kiss, an average of 80 million bacteria are transferred.

Researchers from the Amsterdam-based Micropia Museum and TNO Microbiology and Systems Biology in the Netherlands focused on intimate kissing, “involving full tongue contact and saliva exchange … unique to humankind and common in over 90 percent of known cultures.” The scientists asked 21 couples, aged 17 to 45, who visited the Artis Royal Zoo in Amsterdam to fill out questionnaires—which included queries on the last time they ate, time passed since their last kiss, and how often they engaged in intimate kissing—then had them spit into a tube and swabbed tongues with cotton to analyze what bacteria were already on their tongues and in their saliva. They found that couples had similar bacteria in their saliva and on their tongues, possibly due to shared habits (like smoking), diets, and even toothpaste.

To figure out just how many bacteria make the jump during a kiss, they asked one person from each couple to drink a probiotic liquid that contained specific varieties of Lactobacillus and Bifidobacteria which, according to the researchers, make up 0.15 percent of the bacteria in saliva and 0.01 percent of the bacteria on the tongue on average. After the couple made out, they repeated the spit-and-swab routine; when the researchers analyzed the samples, they found that the quantity of the probiotic bacteria was .54 percent in the receiver’s saliva and .49 percent on the tongue. (Additionally, Remco Kort, lead researcher and microbiologist at the Netherlands Organisation for Applied Scientific Research in Amsterdam, told CBS that “significant difference in height between French-kissing partners can result in a greater exchange of saliva—downward—to the shorter participant.”)

The researchers also found that couples who shared at least nine intimate kisses a day were also more likely to have similar microbes in their saliva—but that they weren’t likely to stick around in the absence of French kissing. “Our findings suggest that the shared microbiota among partners is able to proliferate in the oral cavity,” they write in the study, “but the collective bacteria in the saliva are only transiently present and eventually washed out, while those on the tongue’s surface found a true niche, allowing long-term colonization.”

A steamy tongue kiss might transfer 80 million bacteria, but according to a “Kiss-O-Meter” installed at the Micropia Museum, only 1000 bacteria are transferred by a quick smooch. Pucker up!