The Science of Kissing – MediNick

By CoperNick

Sofia Mao

 

February 14, 2019


 

If you’re reading this article on Valentine’s Day, chances are that you’re surrounded by hearts, chocolates and kissing couples.

Surprisingly, if you were to spend this day among the Mehinaku of Amazonia, you wouldn’t see even a single kiss. Indeed, this particular practice would be regarded as gross and unhygienic. According to anthropologist William Jankowiak, romantic kissing is absent in 5

So, why does the other 4

A possible function of intimate kissing is facilitating bonding, in fact people looking for a long-term partner feel that kissing is more important in the established relationship phase than at the initial stage. A second possibility that kissing plays a role in the assessment of a potential mate, for example the sampling of certain saliva compounds might provide important chemical cues about an individual’s health and genetic fitness. Finally, exchanging saliva might affect our immune system by increasing the chance of contracting orally transmitted diseases. This outcome might seem undesirable, but it is actually useful because some viruses, such as human cytomegalovirus (CMV), are extremely harmful to foetuses’ development but cause less severe symptoms if contracted before pregnancy.

This exchange doesn’t involve only pathogenetic microorganisms, but also the bacteria normally present in our mouth. Bacteria form colonies also on the surface of the skin, intestine and genitourinary tract, and they outnumber human cells in the body by a ratio of 10 to 1. They have been linked to weight-gaining, immune responses and behaviour, so scientists have started studying them as a whole and calling it “microbiota” (you may also find the term “microbiome”, which refers to the genetic makeup of the microbiota). During evolution, microbes established a symbiotic relationship with the host. We provide room and board and in return, they protect us from unwelcome parasites, stimulate our immune system and help our metabolism.

In particular, the predominant genera in the oral microbiota are Streptococcus, Rothia, Neisseria, Gemella, Granulicatella, Haemophilus, Actinomyces, Veillonella, Porphyromonas, and Fusobacterium. Does kissing really affect this bacterial constitution? Remko Kort and other Dutch scientists have conducted an experiment on 21 couples to verify it.

They calculated that a 10-second intimate kiss can transfer up to 80 million of bacteria. This amount was estimated by giving a probiotic yoghurt drink to one of the partners and then asking them to kiss. The drink contained mostly Streptococcus, Lactobacillus, and Bifidobacterium. Since Lactobacillus and Bifidobacterium are not very common in human mouths, we can use their DNA as a marker to study the transfer of bacteria. On the contrary, Streptococcus is the most predominant genus in the oral cavity, so it is difficult to detect changes in its concentration. The researchers collected saliva samples and tongue swabs of the partner who hadn’t received the drink and used real-time PCR to quantify the Lactobacillus and Bifidobacterium DNA before and after kissing. Assuming that all bacteria in the mouth transfer with the same efficiency as the marker and that the kiss contact surface of the tongue covers a total of three tongue swabs, they found that average amount of bacteria transferred per intimate kiss of 10 s is 8∙107.

Cotton swabs for collecting samples

By collecting samples before and after a kiss, without giving the yoghurt drink, Kort discovered that the tongue microbiota is more similar between couple members than between unrelated individuals. By contrast, salivary microbiota changes even in the same individual within 1 minute, and it is still unclear whether these bacteria are indigenous to saliva or are shed by oral surfaces. The level of similarity doesn’t change after an intimate kiss, neither in the tongue nor the saliva.

However, the salivary microbiota becomes more similar when couples kiss at least 9 times per day. Since the flow of saliva is very high and the increased similarity can be detected within 1 h 45 min after the latest kiss, it is possible that some bacteria in the saliva attach to the tongue and colonize it permanently. This would explain why partners share part of their tongue microbiota. On the other hand, the similarity level of tongue microbiota doesn’t correlate to kiss frequency. As a consequence, this similarity may depend on other factors, such as sharing the same diet or oral hygiene practices, or even some genetic traits.

If the last hypothesis were true, it would mean not only that people become more similar because they kiss, but also they want to kiss each other because they are similar.

Also, the experiment led to a funny discovery, that is 7

So, do bacteria really shape our love life? The answer is that they might, but we need further to understand how and to what extent. In the meantime, let’s enjoy the more romantic side of Valentine’s Day.

References

  1. Wlodarski R, Dunbar RI: Examining the possible functions of kissing in romantic relationships. Arch Sex Behav 2013, 42(8):1415–1423.
  2. Kort R, Caspers M, van de Graaf A, van Egmond W, Keijser B, Roeselers G. Shaping the oral microbiota through intimate kissing. 2014;2(1):41
  3. Jankowiak, William & Volsche, Shelly & R Garcia, Justin. (2015). Is the Romantic–Sexual Kiss a Near Human Universal?. American Anthropologist. 117. 10.1111/aman.12286.
  4. Murray, Patrick R, Ken S. Rosenthal, and Michael A. Pfaller. Medical Microbiology. Philadelphia: Elsevier/Saunders, 2013.

 

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