On July 16, 1439, King Henry VI banned mouth-to-mouth kissing in England in an attempt to halt the spread of disease through the kingdom. Needless to say, the unpopular law didn't last very long.
However, it turns out that the overly pious monarch may have been onto something: studies have found that a single snog can transfer as many as 80 million bacteria. Eew.
According to a scientific paper published in the open access journal Microbiome, the oral microbiota in humans are strongly influenced by the people we kiss; particularly when it comes to saliva-based bacteria.
Researchers from the Netherlands took swab samples from 21 couples who were asked to fill out questionnaires on their kissing behaviour including their average "intimate kiss" frequency. In a controlled kissing experiment, a member of each couple also ingested a probiotic drink containing specific varieties of bacteria including Lactobacillus and Bifidobacteria.
The results showed that when couples intimately kiss at relatively high frequencies their salivary microbiota become similar. There was also a lot more of them. The quantity of probiotic bacteria in the receiver’s saliva was found to rise threefold, resulting in a total of 80 million bacteria being transferred during a 10 second kiss.
Similarity indices of microbial communities show that average partners have a more similar oral microbiota composition compared to unrelated individuals, with by far most pronounced similarity for communities associated with the tongue surface. An intimate kiss did not lead to a significant additional increase of the average similarity of the oral microbiota between partners. However, clear correlations were observed between the similarity indices of the salivary microbiota of couples and self-reported kiss frequencies, and the reported time passed after the latest kiss. In control experiments for bacterial transfer, we identified the probiotic Lactobacillus and Bifidobacterium marker bacteria in most kiss receivers, corresponding to an average total bacterial transfer of 80 million bacteria per intimate kiss of 10 seconds.
The study concludes that couples who kiss frequently are a lot more likely to share salivary microbiota due to the frequent bacterial exchange between the mouth and tongue. However, the researchers also note that other factors may contribute to this bacterial similarity, including shared lifestyle habits, environment or genetic factors from the host.
Shaping the oral microbiota through intimate kissing [Microbiome]
This article has been updated since its original publication.