The Human Microbiome of a Kiss. Artwork created by Martin Oeggerli (Micronaut) and dedicated to his 8-years old son Nelson. This image was especially produced for the National Geographic feature article ‘How trillions of microbes affect every stage of our life-from birth to old age’. Soon after its release, the publication became overshadowed by the world’s coronavirus (COVID-19) pandemic – thereby instantly confirming the articles visionary title. Meanwhile (end of April 2020), the crisis has not only severely limited the artists ability to work, but also prohibits him from officially visiting his son who is living in Switzerland, just a stone’s throw away from the rest of the father’s German-based patch-work family.
This picture is based on scanning-electron-microscopy technology, and all colors are manually added by the artist in post-production, to visualize the immense diversity of the microbial community transmitted through a kiss. Whenever we kiss someone on the lips or cheeks invisibly small microbes are exchanged from one person to the other one! The variety of microbes that are part of our microbiome is mind-blowing, and the composition of the invisibly small communities can be the cause of many factors, including the human genetic makeup, diet, age, surroundings, and sexual behavior. Among humans, approximately 90% of cultures have some type of kissing. Usually it is platonic, such as a parent kissing a child. However, in 77 of 168 (46%) of all cultures, it can go as far as intimate (French-) kissing. Recently, a scientific study has revealed that on average 80 million bacteria are transferred to the partner during a kiss of 10 s. Most partners share a more similar oral microbiome compared to unrelated individuals and/or e.g. their kids. But some of the collective bacteria among partners are only transiently present, while others have found a true niche and survive permanently, allowing long-term colonization.With regard to kissing, the researchers report that societies with distinct social classes are usually kissers, while those with fewer social classes, like hunter-gatherer communities, are usually not. Because hunter-gathering societies can act as a window to our evolutionary past, romantic kissing is regarded as a relatively recent (modern) development in human evolutionary history. And, if kissing does affect our microbiome, it’s been doing so for a long, long time. The practice of intimate kissing was already established by the time we built civilizations, and there are numerous allusions to romantic kissing in ancient literature. It’s believed that passionate kissing declined in popularity after the fall of Rome, but made a come-back a thousand years later, thanks to the introduction of courtly love in the Middle Ages. A study recently concluded that romantic kissing actually serves a biological purpose; helping us to evaluate aspects of a potential mate’s suitability or mediate feelings of attachment in a long-term relationship – through chemical taste cues in the saliva – rather than to stimulate arousal and sex. Interestingly, a study hypothesized that French kissing may have evolved to protect pregnant women against human cytomegalovirus (HCMV), a common virus that can cause abnormalities in the fetus. HCMV is easily transmitted through saliva, urine and semen, and the researchers concluded that it would help couples to receive a healthier baby if the virus was already transmitted prior to pregnancy. So, kissing isn’t just about sex. It’s a way to test a partner’s compatibility, birth healthier babies, and swap mouth microbes.