Full story

Just one kiss 'spreads 80 million bugs'

11:15am Monday 17th November 2014 content supplied byNHS Choices

"A single 10-second kiss can transfer as many as 80 million bacteria," BBC News reports. Dutch scientists took "before and after" samples from 21 couples to see the effect an intimate kiss had on the bacteria found in the mouth.

By studying the couples, the scientists discovered the bacteria found on the tongue are more similar among partners than unrelated individuals, but are not correlated with kissing behaviour.

In contrast, the researchers found that for bacteria in saliva to be similar, couples need a relatively high kiss frequency and a short time since their last kiss.

The researchers also estimated that a 10-second kiss transfers 80 million bacteria. These results suggest many of the transferred bacteria are not able to take hold on the tongue.

Some of the media reporting has suggested that this transfer of bacteria that occurs during a kiss is good for us.

The idea is plausible, but is not proven by the evidence presented in the current study. Sometimes, as the song goes, "a kiss is just a kiss".

 

Where did the story come from?

The study was carried out by researchers from the Netherlands Organisation for Applied Scientific Research (TNO) Microbiology and Systems Biology and Micropia, Natura Artis Magistra (Artis Royal Zoo), and VU University Amsterdam, The Netherlands.

It was funded by Natura Artis Magistra and TNO.

The study was published in the peer-reviewed scientific journal Microbiome. This study is open access, meaning it can be read online for free.

The story was well reported by BBC News. But the Daily Mail's coverage was less accurate, as its headline stated: "Kissing for ten seconds passes on 80 million bugs - but it keeps you healthy! Bacteria transferred helps improve immune system". The study made no assessment of immune function, so this statement is unsupported.

 

What kind of research was this?

This was a series of experiments on people that aimed to determine:

  • whether the mouths of kissing partners are colonised with similar bacteria 
  • if the frequency with which couples kiss and the amount of time since the last kiss influences the bacteria present in the mouth
  • the number of bacteria transferred by kissing

 

What did the research involve?

The researchers studied the bacteria in the mouths of 21 couples, including one female and one male gay couple.

The researchers collected saliva samples and samples from the back of the tongue before and after an intimate kiss of 10 seconds. Bacteria were identified by analysing the DNA sequences present in samples.

Couples were also asked to report their last year's average kiss frequency and the period of time since their last intimate kiss.

One of the partners was asked to consume 50ml of a probiotic yoghurt drink containing the bacteria Lactobacillus and Bifidobacteria.

Again, saliva and tongue samples were collected before and after an intimate kiss of 10 seconds. The researchers estimated bacterial transfer after an intimate kiss by tracking these marker bacteria.

 

What were the basic results?

The bacteria found in tongue samples were more similar for couple members than for unrelated individuals. An intimate kiss did not significantly increase the similarity in the bacteria found in tongue samples. 

The bacteria found in saliva were not more similar for couple members than for unrelated individuals, and an intimate kiss did not significantly increase the similarity in the bacteria found in saliva samples.

However, the researchers did see a correlation between the similarity of bacteria found in the saliva of couples and self-reported kiss frequencies, and the reported time since the last kiss.

The researchers estimated that 80 million bacteria are transferred per 10-second intimate kiss.

 

How did the researchers interpret the results?

The researchers concluded that, "This study indicates that a shared salivary microbiota [bacterial flora] requires a frequent and recent bacterial exchange, and is therefore most pronounced in couples with relatively high intimate kiss frequencies.

"The microbiota on the dorsal surface of the tongue is more similar among partners than unrelated individuals, but its similarity does not clearly correlate to kissing behaviour, suggesting an important role for specific selection mechanisms resulting from a shared lifestyle, environment, or genetic factors from the host."

They go on to say that, "Furthermore, our findings imply that some of the collective bacteria among partners are only transiently present, while others have found a true niche on the tongue's surface allowing long-term colonisation."

 

Conclusion

This study has investigated the effects of intimate, or french kissing, on the bacteria found in the mouth.

By studying 21 couples, it found the bacteria on the tongue are more similar among partners than unrelated individuals, but are not correlated with kissing behaviour.

In contrast, the researchers found that for bacteria in saliva to be similar, couples need a relatively high kiss frequency and a short time since their last kiss.

The researchers also estimated that a 10-second kiss transfers 80 million bacteria.

These results suggest that kissing transfers many bacteria, but many of the transferred bacteria are not able to take hold on the tongue.

This is interesting research, but the findings have limited implications. They do not tell us whether kissing is beneficial or not - for example, in terms of causing illness or, conversely, increasing our immunity by exposure to a greater range of bacteria.

Though, of course, a kiss with the right person can be fun.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.

Summary

"A single 10-second kiss can transfer as many as 80 million bacteria," BBC News reports. Dutch scientists took "before and after" samples from 21 couples to see the effect an intimate kiss had on the bacteria found in the mouth.

Links to Headlines

One kiss 'shares 80 million bugs'. BBC News, November 17 2014

Kissing for ten seconds passes on 80 million bugs - but it keeps you healthy! Bacteria transferred helps improve immune system. Daily Mail, November 17 2014

Links to Science

Kort R, Caspers M, van de Graaf A, et al. Shaping the oral microbiota through intimate kissing. Microbiome. Published online November 17 2014

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