What bacteria do to our teeth

Throughout summer I had a not so nice encounter with the least favourite of all doctors – the dentist. While she was tampering with my teeth, I realised I barely know anything about those bacteria that cause this nasty disease known as caries that leads to tooth decay and eventually cavities.

As kids we were always told, brush your teeth and don’t eat sugar before going to bed or the bacteria will come and make little holes in your teeth. So until last week I thought bacteria were the ones MAKING the holes/cavities in our teeth…

Gladly, I did some research and found out that I was wrong (thanks mum!). So, I am going to share with you what I learned, as I guess I was not the only one having wrong ideas and open questions about this issue…

As always, let’s start by defining some terminology.

The upper layer of our teeth is called enamel and consists of inorganic calcium complexes. This is what makes our teeth so white and shiny. This enamel layer undergoes constant processes of mineralisation and demineralisation to keep the tooth intact. During the demineralisation process, calcium is removed from our teeth and this is caused by a low pH.

A low pH means an acidic environment, hence lots of protons (H*), which come from the presence of acids. We can get an acidic environment in our mouth when we eat acidic foods or sugar. Sugar itself is not acidic, however bacteria break down the sugar into acids in a process called fermentation (more on bacterial fermentation here). Bacteria then release these acids into the environment. This is why you should always clean your teeth after having eaten something sweet!

Just seconds after you cleaned your teeth, the saliva in your mouth comes into contact with your teeth and forms a thin layer of saliva on top of your teeth. Saliva consists of many different components, mostly proteins, and plays an important role in our mouth.

Generally, saliva keeps our teeth healthy and free from pathogenic bacteria. It achieves that by washing bacteria and their products away. Saliva keeps the pH in our mouth neutral, it “buffers” the pH, which hinders the demineralisation process. And most importantly, the immune factors in the saliva inhibit and kill pathogenic bacteria and prevent their adhesion to the teeth.

Our commensal mouth microbiota are those bacteria and viruses and microbes that usually live in our mouth. If you want to know more about those bacteria that usually live in our mouth, have a read through this blog article by my friend Justine.

The commensal bacteria have specific adhesins with which they bind to the proteins of the saliva (I describe adhesins in this article). After they attached to the saliva and thus to the tooth, they form a so called dental biofilm on the tooth, also known as plaques. This is the stuff you can feel on your teeth in the morning after waking up (I talk about what biofilms are, here).

The commensal bacteria in the dental biofilm inhibit the settling of pathogenic bacteria and actively kill them. They do this by producing hydrogen peroxide and bacteriocins (which I talk about in this article).

The dental biofilm protects the tooth from pathogens

Commensal bacteria can also produce acids from sugar, but these are weaker acids, which means they do not lead to such a low pH. Plus, they produce compounds that shift the pH and keep it neutral and thus inhibit the demineralisation process of the enamel. So, these bacteria definitely help us keeping our teeth healthy!

And then we have pathogenic bacteria that, as always, are the sneaky ones. Pathogenic bacteria not only bind to saliva on our teeth, but also to sugar and to commensal bacteria. The virulent bacterium Streptococcus mutans can indeed specifically bind to Candida albicans which is a member of our commensal mouth microbiota. Plus, this binding is even enhanced by sugar.

Once pathogenic bacteria attach themselves to the dental biofilm they use the sugar as a building block to increase the biofilm. So now we have a mixed biofilm of good and bad bacteria. What do you think is happening now?

Bacteria build biofilms on teeth

Exactly, a big bacterial fight is about to start! The commensal bacteria try to kill the pathogens to keep our mouth healthy.

But the pathogens have another weapon. They use the sugar to produce strong acids and make the environment very acidic. But our commensal bacteria are not used to such an acidic environment and cannot survive. So this acidic environment eventually kills our commensal bacteria and the pathogens have the biofilm just for themselves.

The now cariogenic biofilm is the onset for tooth decay. The pathogenic bacteria within this biofilm maintain the low pH to keep out the commensal bacteria. This triggers the demineralisation process of the enamel which eventually leads to holes or cavities in the tooth. Hence, it is not the bacteria themselves that “eat” the tooth!

Acidic environment fosters tooth cavities

Generally, this cariogenic biofilm can be understood as a mechanical protection for the pathogens. They also just want to survive (but please not in our mouth!). Such biofilm keeps the pH low, because sugar molecules can diffuse into the biofilm which allows the pathogens to keep making acids. However, the saliva and its inhibitory components cannot enter the biofilm.

So, basically, caries disease can be understood as a shift of our mouth microbiota from the commensal bacteria to pathogenic ones. This leads to a low pH around where the cariogenic biofilm sits, which triggers the demineralisation process of our teeth and results in those nasty cavities everyone is so afraid of.

If you want to know more about what exactly tooth paste does to prevent dental caries, have a look at this blog post.

Take away message from this blog post should be easy: refrain from sugar after hours and keep brushing your teeth! 🙂

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