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  • Writer's pictureP.K. Peterson

Is a Kiss Just a Kiss? Pathogens Transmitted by Kissing

“My upper lip becomes moist, while my lower lip trembles! I shall embrace him, I shall kiss him.”

- Inscription on a tablet from Sippar, Mesopotamia, 1900-1595 B.C.E.

“I’m in love with love, so I’m always taking risks! I think it’s part of love, taking risks.”

- Melanie Laurent, French actress, filmmaker, and singer

Despite the lyrics in the song “As Time Goes By” from the classic film Casablanca, a kiss is not always just a kiss. Two recent scientific articles on kissing (“Humanity’s First Recorded Kiss Was Earlier Than We Thought” in Smithsonianmagazine, and “The ancient history of kissing,” in Science) tell the fascinating story of the origin of romantic kissing and the role that kissing plays in the transmission of human pathogens. In this week’s Germ Gems post, I deal with the latter—the potentially serious consequences of locking lips.

Origin of kissing. The Smithsonian and Science articles both provide an intriguing account of the social and cultural history of kissing. According to these articles, a Bronze Age manuscript dating from 1500 BCE suggests that human romantic sexual kissing originated in South Asia (India), but a substantial body of new evidence indicates that “lip kissing” can be traced to ancient Mesopotamia and Egypt from at least 2500 BCE. Research in this field differentiates two types of kissing, namely, the friendly parental kiss and the romantic sexual kiss. A clay model from Mesopotamia dating to 1800 BCE, shown in the Science article, displays a passionate embrace that clearly indicates these subjects were participants in romantic kissing.

Kissing and the oral microbiome. Romantic kissing is not, however, ubiquitous. In a 2015 study of 168 cultures around the world, researchers found that romantic kissing was popular in only about half of these groups. They also found that where romantic kissing is common, some kissers (and kissees), were known to “pay a price for the experience,” that is, by spreading orally transmitted microbes.

The oral cavity is second only to the gut as a home for one of the largest and most diverse groups of microbes that comprise the human microbiome. Beginning in the 1970s, new genomic technologies began to emerge that allowed scientists to identify microbes (bacteria, archaea, fungi, parasites, and viruses) without the usual culturing. This led to the discovery of more than 99% of microbes that previously weren’t even known to exist.

So far, bacteria are the most studied members of the human microbiome. It’s nearly impossible to accurately determine how many bacteria are present inside the mouth. Some estimates, however, are as high as 20 billion. Additionally, researchers have determined that there are 500 to 650 bacterial species in the oral microbiome.

The contribution of the oral microbiome to health and disease is attracting mounting scientific interest, and its importance extends way beyond periodontal disease. Within the context of understanding how romantic kissing impacts the constituents of the oral cavity, a 2013 study from Comenius University in Bratislava of 12 couples who agreed to kiss each other passionately for at least two minutes showed that the man’s DNA was still present in his romantic partner’s mouth for at least an hour after kissing (a finding the researchers suggested could be useful in crime investigations).

The characteristics of the oral microbiome after passionate kissing aren’t well characterized. Nonetheless, a study at the University of Trento in Italy published earlier this year determined that household members are more likely to share mouth bacterial strains than those of the gut. According to the study, a third of your oral bacteria are shared with other people in your home. The researchers stated that the study “shows how common it is for people to transmit bacteria to others, even those they don’t kiss or have sex with.”

Pathogens transmitted by kissing. Although the bacterial residents of the oral cavity receive a lot of attention, by far viruses capture most of the scientific interest. This was true even before the COVID-19 pandemic erupted in late 2019. In a 2017 review article published in Saliva Protection and Transmissible Diseases, “Viral Diseases Transmissible by Kissing,” the researchers provide a table of “Medical Viruses Detectable in Saliva,” listing them from A (for Adenovirus) to Z (for Zika virus). Shockingly, 29 different virus species were recovered from the saliva of people with evidence of active infections.

Viruses responsible for diseases such as hepatitis viruses, herpesvirus infections (for example, with herpes simplex type 1 and 2, Epstein-Barr virus [EBV], and cytomegalovirus) can be transmitted by kissing, as can a large variety of other viral pathogens, such as Ebola and Zika viruses. EBV, the cause of more than 3 million cases of infectious mononucleosis (“mono”) in the U.S. per year, is often called the “kissing disease.” This is because of the large number of young people who venture into romantic situations involving the sharing of saliva.

Given the immense interest in coronaviruses recently, in particular SARS-CoV-2 (the cause of the COVID-19 pandemic), a number of research studies were conducted to determine whether COVID could be transmitted via kissing. Sadly, the answer is yes.

Early in the pandemic, scientists discovered that SARS-CoV-2 infects cells in the mouth and is present in saliva. While the main mode of transmission of SARS-CoV-2 from person-to-person is via aerosols (coughing, sneezing, and singing), coming into contact with a person’s saliva through kissing or other sexual activity can expose you to the virus. (For more information, see Dr. Carolyn Barber’s article in the March 9, 2022 issue of Scientific American, “When Is It Safe to Have Sex after COVID?”)

Kissing germs goodbye. Now that you know about the large number of human pathogens that can potentially be found within the mouth, it’s important to put this information in perspective. In terms of risk-taking behavior, if you and your partner are healthy and the spirit moves you, “go for it.” You can generally rely on nature to protect you from microbes that come along for the ride when you kiss.

As is the case for the other four body sites that harbor microbes (gut, skin, respiratory tract, and vagina), the oral microbiome is composed of microbes that are either harmless or beneficial to health. If a pathogen finds its way into your oral cavity, these microbes provide a protective service (called “colonization resistance”). And if this mechanism of defense isn’t sufficient, cells of the immune system are called in to rid the oral cavity of foreign invaders.

If you’re ill, or in close contact with someone who’s sick, it’s likely that you are not interested in romantic kissing anyway. Just be patient. As Sam, the crooner in the film Casablanca, reminds us, “The world will always welcome lovers, as time goes by.”

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

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