80 million Bacteria are Exchanged with Each Kiss

Translated by Matteo Bevilacqua, Hunimed student

Those who think that love is the only thing exchanged with a kiss should think again. The “seal” of the poets is a powerful “vector” for the transmission of bacteria.

What happens during the outpouring of the most romantic and passionate effusion?

The Netherlands Organisation for Applied Scientific Research succeeded to count the bacteria jumping from tongue to tongue and published on Microbiome a study that revealed incredible results: 80 million bacteria and 700 different strains are exchanged with each kiss!

No worries, though. It is a gift that is generally harmless. 

“In fact – says Professor Carlo Selmi, Head of Rheumatology and Clinical Immunology at Humanitas – if the kiss is part of our set of social behaviour from the beginning of time, surely there must be some secret in this evolutionary gesture. There might be an explanation for the exchange of bacteria.”

bacteria in kissesHowever, the scientific community is not yet able to provide a definitive account on the phenomenon.

“Yet another fact seems to confirm the importance of this factor – the specialist continued – and that is the fact that 90% of living species have exchanged kisses and bacteria in addition to the alterations of intestinal flora and the skin that characterize inflammatory diseases of the skin and intestines.”

We are made of bacteria!

The experiment was conducted in two phases. In the second, the 21 participating couples were given a probiotic at the start of the experiment. This allowed for a more reliable count to be collected after 10 seconds of kissing. Which was key in order to ascertain the identity of the oral bacterial species after the kiss. Bacteria play a fundamental role in the biological approval process.

“Just think – continues Dr. Selmi – that each of us has more bacteria than cells, and there are crowded colonies in flux, especially those that are the gateways to the outside: the mouth and gastrointestinal tract in particular.”

The general opinion that bacteria is dangerous should be understood more clearly, dropping the unscientific notion that “all bacteria is bad bacteria.”

“Fortunately, scientific research is increasingly producing new and complex discoveries. However, it has yet to decipher the complex difference between the bacteria that live in us and diseases. Imbalances can be harmful most notably with the emergence of autoimmune diseases. For instance, excessive hygiene is a factor that exposes the body, especially the skin, to damage rather than greater security.”



Humanitas is a highly specialized Hospital, Research and Teaching Center. Built around centers for the prevention and treatment of cancer, cardiovascular, neurological and orthopedic disease – together with an Ophthalmic Center and a Fertility Center – Humanitas also operates a highly specialised Emergency Department.