How many bacteria accompany a kiss?

The mouth is infested with microbes, so how many bugs do we swap with a kiss?
02 February 2015

Interview with 

Dr Marcello Riggio, University of Glasgow


Rodin's The KissYour gut begins with your mouth. And Glasgow University's Dr Marcello Riggio looks at the bacteria that live there and what happens when their numbers go off kilter, as he explained to Chris Smith...

Marcello - The mouth is a very, very bacteria rich environment and indeed, compared to other parts of the body, one might be surprised to know that it is really one of the most diverse parts of the human body. There are approximately a thousand different bacterial species in the human mouth although most of us usually carry only up to 75 different types of bacterial species. The mouth is a wonderful place for bacteria to grow. It's got fairly neutral pH and it's got a temperature of about 36 degrees. So, it really is a perfect environment for bacteria to grow in.

Chris - Food on tap as well.

Marcello - Absolutely. Bacteria can gain their food from the saliva of the host and also, food debris from in-between the teeth, which is why is very good to floss your teeth to minimise bacterial metabolism, particularly of the bad bacteria which we mentioned earlier.

Chris - Now, when you talk about flossing of teeth, I think this is an important point because there's an old saying amongst dentists that you don't need to floss all your teeth, just the ones you want to keep. But equally, scientists certainly in New Zealand to start with, now around the world finding that if you have bad gum disease, surprisingly, this is also linked to heart disease.

Marcello - Yes. Gum disease comes in two forms. Essentially, the less severe one is called gingivitis which is an inflammation of the gums and the gums bleed, and affects most of us from time to time. We brush our teeth and we spit out a little bit of blood. But the more advance form of gum disease is known as periodontitis and this is far more severe form if gingivitis isn't treated by good oral hygiene, flossing, use of mouthwashes and brushing their teeth. You will have periodontitis which if not treated, will result in destruction of the surrounding tissues and bone and eventually, tooth loss. Now, this is a particular problem due to the fact that people with periodontal disease, have much higher incidents of more serious disease because the bacteria from the mouth in people who've got gum disease can get into the bloodstream and are carried through the blood to other parts in the body where they can cause life threatening diseases. Things such as heart disease, stroke, diabetes, dementia, rheumatoid arthritis, even premature birth, and endocarditis which is an infection of the underlining of the heart and the heart vowels. These have all be associated with the presence of gum disease and particular, periodontitis the more severe form.

Chris - The thing is though that every time someone cleans their teeth, they're going to get little shower of bacteria going around in the bloodstream, aren't they? So, there must be a difference between that manifestation of a few bacteria going in the blood and this more pronounced form of inflammation that seems to be linked to things like heart disease and stroke.

Marcello - Yes, that's correct. We have what are known as transient bacterium. This is the term that we use for bacteria getting into the blood. But yes, even eating an apple or brushing teeth sets up a very sort of transient and small number of bacteria that do enter the bloodstream. But in small numbers, the immune system can cope quite well with those foreign bacteria in the bloodstream and they're eliminated. The problem is, when you have much more serious diseases such gum disease, periodontitis, and you get larger numbers of bacteria which enter the bloodstream, that tends to overwhelm the immune system. Particularly removing a tooth or having various forms of dental treatment, root canal treatment, etc. These are much more serious levels of bacteria entering the bloodstream which are not eliminated so easily by the immune system.

Chris -   Apart from flossing your teeth, what can you actually do to keep the mouth a healthy place. Can we, for instance do for the mouth what probiotic yoghurts do for the intestine.

Marcello - Yeah, probiotics have been promoted and we see adverts for leading brands on the TV very frequently. Having said that there are some scientific studies that have questioned recently the effectiveness of probiotics. There is evidence to suggest that probiotics can boost natural immunity because they introduced good bacteria both into the gut and also, into the oral cavity. It's far more difficult to have a stable introduction of the good bacteria present in probiotics, for them to actually colonise or set up home in the oral cavity.

Chris - Now obviously, Valentine's day is not far away. When you snog someone, are you quite literally and it's quite funny because our producer Georgia has written here, "Are you doing a foreign exchange programme for your microorganisms with the person that you're getting jiggy with?"

Marcello - Very much so and I don't want to put people off kissing but a recent study did show that around 80 million bacteria are transferred in a single 10-second intimate kisses the researchers succinctly put it. What these researchers did, they looked at couples and they gave one member of the couple a probiotic drink containing easily identifiable bacteria. They looked at the populations of bacteria on the tongue and saliva samples before and after the kiss. They showed that the bacteria in the saliva of the partner who was accepting the saliva in effect, those bacteria within a probiotic drink, rose three fold whereas the bacteria on the tongue remain much more stable. And also, what they found was, the more often a couple kiss, the more similar the bacteria that they shared in the saliva samples. Obviously, if you live with somebody, there are various mechanisms that exist for sharing of bacteria. You've got shared lifestyle and similar perhaps dietary and personal care habits.


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