The Other Microbiome – The Microbes Living in Our Mouth


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Surprisingly, the microbiome of the mouth is still mostly uncharted territory compared to that of the gut. Here, at the entrance to our digestive tract, live up to 10 billion bacteria – that’s more than humans on our planet! In addition, there are fungi, viruses, amoebae, and other microorganisms.

I often mention in my lectures that we humans have always longed for peace, even though the threat of war with outside intruders is always present in our bodies. Every moment our protective species are fighting together with our immune system against unwanted invaders.

Our teeth are covered on their surface with a so-called “pellicle”. The role of this film-like deposit, which measures only 0.5-1 µm, is ambivalent. On the one hand, it protects us by contributing to the remineralization of the tooth. On the other hand, without this protein film, bacteria would not be able to adhere to the smooth surface of the teeth creating plaques or the so-called oral biofilm.

The bacteria don’t just cavort on our tooth surfaces, they are organized into what is called a biofilm. Something similar to the history of our civilization is happening here: just as people banded together in cities to be better armed against storms or attackers, microorganisms brave toxic substances and other aggressors in close collaboration.

Like all biofilms, plaque develops on a moist surface – that of the tooth. Initially, the so-called pioneer germs attach themselves to the pellicle with the help of hair-like extensions, the so-called pili, and fimbriae.

The bacterial “lawn” first grows in width and eventually in thickness, forming bacterial colonies. But the microbes are even more resourceful: just as we humans do when building cities, they also build special water channels throughout the biofilm that can carry food from the outside to the inside and the toxins produced by the bacteria from the inside to the outside.

If you look more closely, these biofilms are only a small part bacterium. Rather, we are dealing with a fine network of complex carbohydrates (so-called exopolysaccharides) that provide shelter for the biofilm inhabitants. They are produced by the bacteria and perform many tasks: Not only do they house the bacteria, but they can also store nutrients, play a role in regulating pH and in communication between bacteria.

So, these are the cities where our bacteria live, eat, adjust the desired acidity and communicate with each other, and with our human cells. The transition between the hard tissue of the tooth and its soft tissue, i.e., the periodontium, is an ideal habitat for bacteria. This is the only place in our body where the epithelium – the uppermost cell layer of our skin and mucous membranes – is broken through. Otherwise, this is only the case when we injure ourselves.

This spot is not always optimally accessible with a toothbrush – especially in the area between the teeth. Due to a complex process, tartar is formed in these places, and bacterial strains can settle even more easily on these rough areas.

If these bacteria remain in a healthy balance, it does not pose a danger to our health. The biofilm grows in thickness, but is eventually removed by the tongue, saliva and – possibly – the toothbrush.

It only becomes dangerous when the relative abundances of the oral microbiota become altered. This dysbiosis happens gradually: First, so-called bridge germs settle, paving the way for the aggressive ones.

Naturally, our body fights back – with engagement of the immune system. I do not propose to go into detail here about the complex mechanisms that are set in motion in our body during an inflammation. In any case, these are war-like conditions, through which our body defends itself against the invasion of dangerous microorganisms.

As long as this inflammation remains confined to the gums, it is called gingivitis, a reversible condition. If you improve your oral hygiene and clean all these inflamed areas as thoroughly as possible, the inflammation will spontaneously recede.

Exactly when and why the inflammation begins to interfere with the metabolism of our jawbone is still being investigated. Many factors interact here, i.e., the process is multifactorial, as is the case with many other diseases of our body.

In gingivitis, the connection between bone and tooth is intact. Bone formation and degradation at the junction between tooth and bone continues. We have a balance between build-up and breakdown. With periodontitis, however, there is a shift in this balance toward bone breakdown.

I mostly explain it this way: the body is suffering from the constant stress of chronic inflammation in the mouth. To try to get rid of it, it pulls back to eliminate the source of the inflammation, the tooth.

So, our bone retreats in flight from the attackers. As a result, the periodontal pocket deepens. In its depth, the dangerous bacteria have better conditions to survive, including the oxygen-free environment, and protection from any interference by toothbrushes, and antibacterial agents, such as toothpaste or antiseptic mouthwash. Over time, eubiosis (the balance of microorganisms living in a person’s healthy mouth) develops into dysbiosis (an imbalance).

If you add up all the inflammatory areas in the mouth of a fully dentate person with periodontitis, it is a wound the size of your palm. A wound that many of us often carry around unnoticed and through which bacteria can invariably enter our bloodstream.

According to a 2012 CDC report, 47.2% of U.S. adults aged 30 and older suffer from some form of periodontal disease. Among the over-65s, the figure is already 70.1%.

Unfortunately, this chronic sore is far too rarely diagnosed by primary care physicians, internists, and other specialists because it is not within their area of expertise. Yet the treatment of periodontitis is not expensive and its prevention through efficient oral hygiene is very successful.

The links between oral and general health have been known long before knowledge of the microbiome. The best documented is the one between diabetes and periodontal disease, but cardiovascular and respiratory diseases, rheumatoid arthritis, cancer, and Alzheimer’s disease – among others – are also related to our oral health.

In summary, with all the hype about the gut microbiome, we shouldn’t forget the important role that oral hygiene and a healthy mouth microbiome play in health and chronic disease.

Dr. Diana Kessler studied dentistry at the University of Heidelberg in Germany and has worked as a dentist in private practice since 1991, Dr. Kessler has seen patients at the diabetes outpatient clinic at the St. Josef Hospital in Heidelberg since 2012, and has lectured on the topic of oral and general health, and on the role of the oral microbiome in chronic disease. She is the author of the book, Gesund durch den Mund.