A quick peck? Fine. Anything else, and you could be getting a lot more than you bargained for.

By Samantha Zabell
Updated November 18, 2014
Couple kissing
Credit: Tim Robberts/Getty Images

There is officially no such thing as an “innocent kiss,” at least according to new research published in journal Microbiome. Turns out, a kiss on the lips is a gateway for the transfer of millions of bacteria.

Researchers from Micropia and TNO in the Netherlands studied 21 couples, and surveyed them on their “kissing behavior,” including how frequently they kissed their partners. They also swabbed each partner’s tongue to look at oral bacteria.

You’ve heard people say that couples start to look like each other, right? This study takes it to a whole new level. Partners who kissed more than nine times per day showed similar communities of oral bacteria.

To see how much bacteria was actually transferred during a kiss, researchers asked one member of the couple to consume a probiotic drink that contained very specific bacteria. They then shared an “intimate” kiss with their partner—more than 10 seconds—and after some calculation, found the magic number was 80 million. 80 million bacteria were transferred from one partner to the other in just 10 seconds.

“Intimate kissing involving full tongue contact and saliva exchange appears to be a courtship behavior unique to humans and is common in over 90% of known cultures,” lead author Remco Kort from TNO said in a statement. “Interestingly, the current explanations for the function of intimate kissing in humans include an important role for the microbiota present in the oral cavity, although to our knowledge, the exact effects of intimate kissing on the oral microbiota have never been studied… it turns out, the more a couple kiss, the more similar they are."

Before you get grossed out, keep this in mind: That bacterial exchange from kissing may actually help to boost our overall immunity, assuming you’ve chosen a healthy partner.