Kissing Transfers Germs!

Eighty million bacteria are transferred in a single kiss, according to a new paper published in the journal Microbiome. A research team from the Netherlands Organisation for Applied Scientific Research concluded that a 10-second kiss is all that is needed for this massive transfer of bacteria from one person to another to take place.

The researchers were interested in understanding the degree of bacterial transfer because the different microbial communities that exist within an individual can be changed by a number of different aspects, including diet,  genetic make-up, present and past surroundings, sexual behavior and age. The results indicated that, on average, people who were partners shared a more similar oral microbiota that people who were strangers did.

The researchers studied 21 couples. They were interested in people who kissed one another on a regular basis so that they could compare the bacteria as it was swapped from tongue to tongue and into saliva. More specifically, the study authors were interested in heavy and intimate kissing, informally and widely known as “the french kiss.”

In fact, french kissing is so common that it is a method of kissing prevalent in 90 percent of known human cultures. Mouth-to-mouth contact is also an activity seen in non-human species such as fish, primates and birds, but the type of intimacy as shown in the french kiss is something that is unique to humans. The researchers wrote that this saliva-exchanging activity is thought to be the result of evolving courtship behaviors.

There have been a number of hypotheses proposed as to why humans kiss. Some researchers have suggested that intimate kissing is a behavior that allows partners to assess another person’s suitability. Others have argued that mouth-to-mouth contact is an absurd and rather risky way to garner information on the quality of a potential mate, and that kissing could actually be an activity that functions  to protect women from Human Cytomegalovirus (HCMV) as carried by men. HCMV is a type of herpes that can be transmitted to embryos. HCMV is present in saliva, and they theory is that women can build immunity from the virus when they swap saliva with a man who carries it.

Earlier this year, another study demonstrated that people who live in the same house share more of their microbiota than people who do not live together. The researchers were interested to see if a single kiss was enough to affect a person’s oral microbiota, but also if the frequency of kissing made a difference.

Using a self-reported kissing history questionnaire, the researchers were able to examine correlations between kissing frequency and similarities in oral microbiota. Couples were asked to report, on average, their kiss frequency for the past year. Interestingly, 74 percent of the men questioned reported a higher kissing frequency than their female partner did. Men estimated on average that they took part in 10 kisses per day, whereas their female partners estimated an average of five. The researchers noted that previous studies have concluded that when it comes to assessing their own sexual activity, men often over-estimate.

In order to evaluate the amount of bacteria that a single kiss might transfer, the participants were asked to drink a probiotic yoghurt containing common yoghurt bacteria. The researchers were able to use the bacteria in this yoghurt drink as a marker of transfer. They concluded that a shared salivary microbiota is only present in couples who kiss often. The action of intimate kissing does transfer a large number of bacteria, but this is a short-lived effect and the bacteria appear to get washed away very soon after kissing ceases.

According to study author Remco Kort, being in a relationship with another person for a long time leads to the bacteria on the tongue changing to become more similar to each other’s bacterial passengers.Further research is needed to investigate the implications of this shared bacterial pool. In the meantime, Kort says that interested couples can have their bacteria accessed for the number of microorganisms that they exchange on a regular basis at the Micropia museum in Amsterdam,

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