What’s in his kiss? Apparently, it’s a lot more than you might think. According to a new study published in Microbiome, a passionate kiss transfers about 80 million bacteria between partners.

The study involved 21 couples and measured how many microbes they exchanged during an average make-out session. “In a single kiss of 10 seconds, 80 million bacteria are transferred, on average, from one person to the other,” says Remco Kort, principal scientist with the Netherlands Organization for Applied Scientific Research, which collaborated with Amsterdam’s Micropia Museum for the study.

Study scientists arrived at the number by asking one partner to drink a yogurt beverage that contained lactobacillus and bifidobacteria, bacteria that are not usually found in the mouth. The couples then proceeded to kiss for 10 seconds. The researchers took samples from the tongue and saliva of the couples before and after they smooched. The 2 yogurt bacteria together usually make up about 0.15% of bacteria in human saliva and 0.01% of bacteria on the tongue. However, after the kiss, the lactobacillus and bifidobacteria concentrations in the partner who did not drink the yogurt rose to 0.54% in saliva and 0.49% on the tongue.

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Likewise, the researchers found out that couples who kissed more than 9 times a day were more likely to have similar saliva microbes than strangers. In other words, “kissing affects the microbes living in your mouth,” says Kort.

But before you go running to disinfect your mouth, kissing and sharing your partner’s bacteria may actually help you become healthier. Sharing mouth bacteria diversifies a person’s bacteria profile, which can help bolster the immune system to fight off illness. Andrea Azcarate-Peril, a microbiologist at the University of North Carolina School of Medicine, says researchers may even be able to go one step further and transplant beneficial oral bacteria from one person to another. “Maybe we could transplant the mouth bacteria of a person who doesn’t get cavities to one who often does,” she said.

Want to know how many microbes you and your partner swap? A museum in Amsterdam called Micropia, which devotes its time to the world of micro-organisms, has a “Kiss-O-Meter” that tells you how many and what types of microbes you and your partner share. Just stand on the stage and pucker up!


  1. Bruzek A. What’s in his kiss? 80 million bacteria. NPR website. November 17, 2014. http://www.npr.org/blogs/health/2014/11/17/364054843/whats-in-his-kiss-80-million-bacteria.
  2. Kort R, Caspers M, van de Graaf A, van Egmond W, Keijser B, Roeselers G. Shaping the oral microbiota through intimate kissing. Microbiome. 2014;2:41. http://www.microbiomejournal.com/content/pdf/2049-2618-2-41.pdf.
  3. Van Marsh A. Love bugs: how many bacteria in a single kiss? CBS News website. November 17, 2014. http://www.cbsnews.com/news/kissing-couples-transmit-80-million-bacteria.