Bacteria that live in the mouth: characteristics, types and functions

Bacteria are everywhere, even if we are not able to see them at first glance. Various studies estimate that, of the 550,000 million tons of organic matter present in the medium in the form of carbon, 70,000 million correspond to the bacteria that live in the ecosystems. In other words, the bacterial mass of soils and waters contains 15% of all organic matter on the planet, a value only surpassed by plants (450 gigatons, 80%).

However, you don't have to go on an astronomical scale to realize that bacteria are everywhere. If you are reading this from your mobile phone, you will be surprised to know that there are more than 25,000 bacterial units for every 6.5 square centimeters of screen (more than a toilet). Another shocking fact: between the elbow and the tips of the fingers are up to 10 million bacteria, all of them living on our epidermal surface.

With all these figures, it is more than clear to us that there are bacteria everywhere. The human being is an open system, and as such, it is not free from the colonization of microorganisms in different parts of the world. tracts that exchange substances with the environment, such as the nasal passages, stomach, esophagus, eyes, anus or the mouth. In order to explore a terrain of great interest, today we explore briefly

bacteria that live in the mouth.

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General concepts about the microbiome

Before looking at the microorganisms resident in the mouth, it is necessary to lay down a series of bases as regards our relationship with bacteria. The term "flora" is often used to designate the colonies present in our tracts and mucous membranes, but these biological units have nothing to do with the kingdom Plantae. Thus, the most appropriate terms are "microbiome" or "normal microbiota".

The human microbiota is divided into two categories:

• Autochthonous microbiota: is one that encompasses the microorganisms that inhabit the human system for a long time. Due to its close relationship with the host, it has evolved with it through symbiotic mechanisms.
• Allochthonous microbiota: these are microorganisms that do not require the host to live, so they can be found in any ecosystem. In any case, they can end up on the surface or tracts of the human being temporarily.

The autochthonous microbiota can be commensal or symbiont, but never harmful in a normal situation. What a bacterial community that lives in equilibrium with humans wants the least is to cause pathogenesis, because if it dies, the population of microorganisms also disappears. Unfortunately, some symbiotic or commensal bacteria can become a problem, especially in patients with immunosuppression (AIDS) or concomitant diseases (such as cancer).

In addition, the microbiota can be latent (remains relatively constant throughout life) or transitory (It presents continuous fluctuations according to the age of the host, the environment or the season). In general, the essential microorganisms for the life of the human being are those autochthonous and latent.

What bacteria live in the mouth?

The terms "oral microbiome", "oral microbiota" or "oral flora" refer to the same thing: the microorganisms that inhabit the oral cavity of the human being. However, rather than the number of bacteria per square centimeter of tissue, what is usually estimated is the bacterial genome present in the samples. In other words, the richness of living beings that inhabit our mouth is determined by the detection of genes from different species and strains, not by direct observation of them. This is so because, for example, some described microorganisms cannot be cultured in the laboratory.

The oral cavity contains one of the most intricate and complex microbiomes in the entire human body, perhaps only surpassed by the intestines and the digestive processes that take place there.

To give you an idea, it is estimated that one milliliter of saliva contains 10 to 8 microbial units, which are distributed in about 700 taxa of prokaryotic organisms. Generally, a healthy "oral flora" is believed to be made up of about 200 bacterial species.

The oral biome is extremely diverse for the simple reason that there are many small-scale niches to exploit. We have the saliva, the surface of the teeth, the gums, the tissues and mucous membranes of the inner face of the cheek, the palate, the sub and supragingival plates and the tonsils, among many other structures. Each of these surfaces favors a different composition of the microbiome.

According to the Human Oral Microbiome Database (HOMD), only 57% of the species of bacteria that inhabit our mouth have been officially described and cataloged, 13% have been cultivated but not classified and 30% have not even been cultivated in the laboratory environment. Yes, although it may seem strange these days, we do not even know for sure which microorganisms help us survive.

The main bacterial groups in the mouth

As the study "Oral microbiome: Unveiling the fundamentals" indicates, oral bacterial communities have been assembled into 185 genera and 12 different phyla. These phyla are the following: Firmicutes, Fusobacteria, Proteobacteria, Actinobacteria, Bacteroidetes, Chlamydiae, Chloroflexi, Spirochaetes, SR1, Synergistetes, Saccharibacteria (TM7) and Gracilibacteria (GN02).

In any case, it should be noted that the fluctuation of many of these microorganisms is changeable, that is, some are allochthonous, transient, or both. The consumption of food, the use of drugs (such as tobacco, since the smoke enters through the mouth), oral hygiene and changes in the pH of the oral environment can cause rises or falls in the numbers of colonies of a given species. Therefore, this medium is quite volatile and difficult to study.

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The functions of the oral microbiome

The oral microbiome usually appears in the form of a biofilm, that is, as an organized “microbial mat” adhered to a surface or substrate, in this case the teeth, mucous membranes and oral tissues. Once this biological community is established, the components secrete a protective extracellular matrix, which keeps its members even more united. This is what is known as “dental plaque” and, although it must be controlled with constant brushing, it is not bad in itself for the body.

The formation of the oral biofilm is essential to maintain the homeostasis of this environment and to avoid the pathogenesis of other microorganisms.. The premise is simple: if there is already a bacterial colony settled on a given surface, it is much more difficult for another potentially pathogenic colony to begin to proliferate in the same place. This is a clear example of symbiosis (even if it is involuntary), since as the space is already occupied, the host is indirectly protected from unnecessary invasions.

Furthermore, in the oral community there are bacteria that express the enzyme nitrate-reductase, thus converting nitrate into nitrite when compounds containing it enter the mouth. This means that, together with the enzymatic action of saliva, digestion begins from the oral cavity thanks to the help of bacteria.

Beyond all this, it has also been shown that these microorganisms are essential for the specialization of the immune system in early stages of development, to keep pro-inflammatory responses at bay and even to actively fight other pathogens. Some bacterial populations are capable of secreting bactericidal substances into the environment, which protect us directly from the invasion of harmful germs.

Resume

As surprising as it may seem in the rise of scientific innovation and knowledge, there is still a myriad of things that we still have to know. The lines drawn up are a clear example of this, because even though we know that the oral microbiota is essential for our systemic well-being, we have not yet been able to classify almost half of the microorganisms that there inhabit.

However, what we are clear about is that, as a species, we are nothing without our microbes. The bacteria that inhabit our systems are essential for fighting disease, training the immune system, digesting food, and much more. Without them, life as we know it today would not be possible.