Researchers in the Netherlands recruited 21 couples to share an "intimate kiss," defined as involving "full tongue contact and saliva exchange." The participants' saliva was tested before and after, with results showing that a 10-second French kiss enables the transfer of at least 80 million bacteria.
Researchers asked some participants to drink a probiotic yogurt drink prior to the kiss to simplify the process of confirming the movement of unique bacteria from one mouth to another.
"We identified the probiotic Lactobacillus and Bifidobacterium marker bacteria in most kiss receivers, corresponding to an average total bacterial transfer of 80 million bacteria per intimate kiss of 10" seconds," the researchers wrote in their new study -- published this week in the journal Microbiome.
While a single kiss wasn't enough to significantly alter the makeup of a person's microbiota or microbiome, the researchers did find that the microbiomes of couples were more similar the more frequently they kissed. The microbiota is the localized collection of micro-organisms inhabiting the body. In this case, researchers were looking at the oral microbiota, the collection of bacteria living in the mouth.
"French kissing is a great example of exposure to a gigantic number of bacteria in a short time," study author Remco Kort, a researcher at the Netherlands Organization for Applied Scientific Research, told the BBC News. "But only some bacteria transferred from a kiss seemed to take hold on the tongue."
"Further research should look at the properties of the bacteria and the tongue that contribute to this sticking power," added Kort. "These types of investigations may help us design future bacterial therapies and help people with troublesome bacterial problems."