What is the role of the intestinal microbiota in the maintenance of health? It is way more important than we think.
In our intestines live billions of bacteria weighing more than our brains. This vast ecosystem we live in symbiosis with is called the intestinal microbiota.
As a whole, there are many exchanges between the intestine and the brain through what is called the “intestine-brain axis.” Abnormalities of the microbiota can lead to potentially life-changing consequences linked to brain activity, energy balance, and immunity.
This explains the role of the intestinal microbiota in the maintenance of good health.
Gut microbiota is not only active in our digestive process, but it is also essential for our immune system. It also acts as a protective barrier for our intestinal wall against possible digestive disorders and diseases.
It would probably be possible to live in a world without microbes, but this would inevitably impact our basic physiological functions.
Read on to understand about the different relationships between this intestinal microbiota and our vital functions.
Table of Contents
- 1 What is the role of the intestinal microbiota in the maintenance of health? Introduction
- 2 The relationship between the gut microbiota and our brain
- 2.1 The intestinal microbiota is essential for brain development
- 2.2 Our brain can modify the composition of the microbiota
- 2.3 Microglia: immunological cells at the heart of our brain
- 2.4 A network of neurons wraps itself around our digestive tract
- 2.5 The microbiota: a link with our cognitive abilities and moods
- 2.6 The intestinal microbiota interferes with the function of intestinal enteroendocrine cells
- 2.7 The intestinal microbiota and our social life
- 2.8 A disturbing example of behavior modification by a microorganism
- 3 The intestinal microbiota and our defense systems
- 4 The intestinal microbiota is a weight regulator
- 5 What is the role of the intestinal microbiota in the maintenance of health? Conclusion
What is the role of the intestinal microbiota in the maintenance of health? Introduction
We need to associate the microbiota with the proper functioning of our body as the bacteria that make up our microbiota play a crucial role in our well-being and health.
Intestinal microbiota is composed of billions of bacteria and includes more than 90% of all the bacteria in our body.
It is essential to have a balanced microbiota because it performs several vital functions in our body.
It has a protective role in defending it against possible disorders and is also involved in metabolic functions. It allows the absorption and production of nutrients and is essential to maintain a good immune system.
The relationship between the gut microbiota and our brain
There are many exchanges between the intestine and the brain through what is called the “intestine-brain axis.”
This axis uses hormonal, immunological, and neurological signals to connect the intestinal microbiota and its metabolites with our brain, and it works in both directions.
The intestinal microbiota is essential for brain development
To know how our brain would develop without the intestinal microbiota, let’s look at an experiment on mice.
Researchers observed the behavior of young, germ-free mice raised in a sterile environment and devoid of intestinal microbiota.
They discovered that they were constantly on the move to calm a state of permanent anxiety and that they had wildly exaggerated reactions to the slightest stress.
In addition, they also observed risky behaviors and deterioration in spatial memory and learning abilities.
Of course, in theory, one could assume that these behaviors were induced by the particular rearing conditions.
However, at the age of 6 weeks, the researchers either gave them an oral probiotic, a bacterium called Bifidobacterium longum infantis or performed a gut flora transplant.
The results showed that following intestinal flora development, these behavioral abnormalities were minimal and the importance of the gut microbiota in the early development of a normal brain was thus revealed.
Tip: While on the topic of probiotics, you may be interested to learn about a number of natural probiotics: Best probiotic foods for gut health: Ultimate list of natural probiotics
Our brain can modify the composition of the microbiota
Our brain has several actions that can modify the composition of the microbiota:
- By acting on the motricity of the digestive tract and on the production of mucus,
- By changing the intestinal wall’s immunological activity or the enteroendocrine cells’ secretions.
It should also be noted that stress can also impact the composition of the microbiota and the quality of the intestinal mucosal barrier.
Studies in rats have shown that violent stress occurring very early in life can induce permanent damage to the intestinal barrier with potentially serious long-term consequences.
When we are stressed, we produce hormones called “catecholamines .”These can reach the intestinal wall and alter the communication signals between the bacteria.
They can also activate the virulence genes of certain bacteria and can therefore become dangerous for our organism.
Our brains are also involved in the amount and choice of food and the amount of alcohol we consume. Therefore, our brain can also change the composition of the microbiota.
Indeed, the food acts as a soil (called prebiotic) that will promote the proliferation of germs necessary for their digestion.
Microglia: immunological cells at the heart of our brain
When our brain is still in an “embryonic” state, it is still composed of a few neurons when macrophages (which are cells of innate immunity) detach from the yolk sac and come to mix with the nerve cells.
These macrophages will then form a tissue intimately linked to the nerve cells, called microglia, which infiltrates the entire brain and spinal cord.
As a result of their defense missions, macrophages can be induced to secrete antibacterial substances. These will be likely to cause damage to the surrounding brain tissue making things worse.
This shows that innate immunity is effective but can also be a bit brutal.
Microglia play an essential role in the defense of the brain but also in its maturation and functioning.
It has been linked to the initiation and development of degenerative diseases (such as Alzheimer’s and Parkinson’s) or autoimmune brain diseases.
Research has shown that the absence of gut microbiota in germ-free mice is associated with immunological hyperreactivity of microglia in the presence of bacterial endotoxins.
According to these studies, hyperreactivity is much more pronounced in females than males.
In germ-free pregnant mice, the lack of gut flora affects male pups. They are born with immature microglia with reduced immunological and metabolic capacities.
On the other hand, the microglia of the small females born from these germ-free mice were normal.
In summary of this research, the absence of intestinal microbiota in the animal or its mother can have a marked impact on the functioning of the microglia and will differ depending on the sex of the animal.
These studies showed the first understanding of the role of environmental factors, particularly alterations in the gut microbiota, in our brain development.
In addition, gender differences in neuropsychiatric illness were evident. Indeed, men are at greater risk than women for early illnesses such as autistic disorders and early and disabling forms of schizophrenia.
On the other hand, it was found that women suffer more often than men from diseases that occur in adolescence or adulthood, such as depression and multiple sclerosis.
A network of neurons wraps itself around our digestive tract
Observed as simple networks of nerves intended to control the various functions of the digestive tract by the brain, our view of these nervous structures has evolved to the point of considering them as a “second brain.”
In fact, they are primitive neurological structures in charge of ensuring the vital functions of the first complex multicellular organisms: worms whose only nervous structures were wrapped around the digestive tract.
Nature has preserved these structures throughout evolution. These are called the “enteric nervous system” (ENS), to distinguish them from the central nervous system (CNS) that evolved at a later date.
The ENS is composed of millions of neurons associated with glial cells. These neurons are grouped into multiple small clusters, called “ganglia,” connected by a dense network of axons.
Two major ganglionic systems group all the neurons of the ENS:
- The myenteric ganglia which are located between two layers of muscle fibers of the digestive wall
- And the submucosal system located, as its name indicates, under the mucosa.
These two complex structures, linked together, are related to the central nervous system but keep an autonomy allowing them coordinated actions.
The ENS is connected to the brain by the vagus nerve, which sends information to a particular circuit in the brain, the hypothalamus-amygdala-cortex circuit, which corresponds to the emotional circuit.
The enteric nervous system synthesizes about twenty neurotransmitters, chemical substances ensuring communication between nerve cells.
Two of these neurotransmitters predominate:
- Dopamine, 50% of which is secreted in the ENS
- And serotonin, involved in regulating our emotions and moods, of which more than 90% is secreted by the neurons of the ENS.
This explains the critical role of this nervous structure in the genesis and/or regulation of our emotions, our moods, and perhaps even our dreams.
The ENS is very close to the billions of gut microbes, some of which are capable of synthesizing neurotransmitters or their chemical precursors.
Under these conditions, it is conceivable that the composition of the microbiota and the nature of the substrates provided by the diet can modify the functioning of the ENS and even the central nervous system (CNS).
It is interesting to mention that some intestinal bacteria participate in the secretion of neurotransmitters. Here are some examples:
As mentioned earlier, serotonin plays an essential role in our brain’s cognitive functions.
According to some studies, many bacteria contribute to the synthesis of this neurotransmitter, whose concentration decreases by 60% in germ-free mice.
These interactions between central nervous system neurotransmitters and the gut microbiota explain the abnormalities in brain development observed in germ-free animals.
They also allow us to understand the impact of intestinal dysbiosis on brain function and on the risk of inflammatory or degenerative diseases of the central nervous system.
Gamma-aminobutyric acid (GABA)
GABA is the main inhibitory brain neurotransmitter. Indeed, it is in charge of stopping the “excitation” of the activated neurons.
So it plays a calming role and is used by certain tranquilizers to reinforce its activity.
Several intestinal bacteria (belonging to the Lactobacillus, Bifidobacteria, and Bacteroides families) produce GABA.
By decreasing the concentrations of these bacteria, there is a depletion of the intestinal flora, reducing GABA concentrations in the intestine and the brain.
Studies have identified a bacterium that had been detected by RNA sequencing techniques but had never been able to be cultured until now.
This bacterium requires the presence of Bacteroides fragilis, a GABA-producing bacterium on which it feeds exclusively.
GABA-synthesizing bacteria and GABA-eating bacteria contribute to regulating the activity of neurons using this neurotransmitter.
Glutamic acid or glutamate
Do not confuse it with its sodium salt (sodium glutamate), which is a food additive!
Glutamate is the main excitatory neurotransmitter of the central nervous system.
It plays a role in learning and memorization and acts through receptors whose concentration in the hippocampus is decreased in germ-free mice.
The intestinal microbiota interferes with the function of intestinal enteroendocrine cells
The intestinal enteroendocrine cells represent 1% of the digestive epithelial cells. However, they play an essential role in the secretion of multiple hormones with a local or general action.
Very specific, each enteroendocrine cell secretes only one hormone or neuromediator.
The hormones secreted by these cells regulate digestive secretions and motricity, the absorption of fats, the transformation of our food, and the impermeability of the intestinal barrier.
Moreover, the neuromediators secreted by these cells act directly on peri-intestinal nervous structures from the enteric nervous system connected by the vagus nerve to our brain.
The microbiota can establish a dialogue with the enteroendocrine cells through substances produced by food fermentation.
The nature of these substances will depend on:
- Either the diet which determines the substrates arriving in the intestine where they will undergo fermentation,
- Or the intestinal microbiota composition will direct the types of substances produced by the fermentation.
The fermentation of fibers produced by short-chain fatty acids has different properties. Indeed, it can be:
- An energy source for the colonic cells.
- A substrate for the synthesis of cholesterol.
- A substrate for the synthesis of sugar by the intestine and the liver.
Some short-chain fatty acids behave as signals capable of modifying the functioning of enteroendocrine cells by acting on receptors located on their surface.
By directing fermentation, the intestinal flora can act indirectly on regulating the body’s major metabolic pathways.
It can also modify the appetite by stimulating the activity of specific entero-endocrine cells. It can also act directly on the brain cells of the satiety centers by producing acetate (an active short-chain fatty acid) on these cells.
Several studies have shown that the microbiota could be involved in our social behaviors.
According to this research, germ-free mice have impaired social relationships although they can be restored by fecal transplantation after weaning.
This transplant partially restores social relationships, but in adulthood, the mice will remain unable to differentiate between their behavior towards familiar and unfamiliar mice.
The administration of antibiotics, which damages the intestinal microbiota, disrupts the social relationships of rodents. Social disturbances were also observed under the same conditions in zebrafish.
A disturbing example of behavior modification by a microorganism
Research was conducted on behavior modification by a microorganism, concerning an infection related to a parasitic microorganism, Toxoplasma gondii.
When this parasite is accidentally ingested, it penetrates the intestinal wall cells and then spreads to many cells in the body.
This parasite will remain there for a long time, taking refuge in the cells least exposed to immune monitoring, namely retinal, brain and muscle cells.
This intracellular parasite is responsible for toxoplasmosis in humans, a disease that is generally benign in the absence of an immune deficiency but can cause fetal malformations during pregnancy.
Capable of infecting all homeothermic animals, Toxoplasma gondii can only multiply in felids, i.e., cats. For this reason, it is recommended that non-immune pregnant women avoid contact with cats and their feces.
Studies have observed that rats infected with Toxoplasma gondii lost their innate fear of cats and even showed sexual arousal in the presence of their urine. In addition, infected rodents were more agitated than uninfected ones.
We know that cats are very attracted to movement, and the characteristics acquired by the infected rats will induce them so as to become easy prey. Consequently, by finding its natural host, the parasite will be able to ensure its descent.
At first sight, this experiment is very worrying because it shows that a rustic, tiny, unicellular parasite can modify the behavior of a small mammal and even cause dangerous behavior changes.
Other studies conducted with chimpanzees have also confirmed this result with research linking chimpanzees infected by Toxoplasma gondii with leopard urine, their natural predator.
Men infected with Toxoplasma also find the smell of cat urine attractive, unlike uninfected men.
These facts could make us think that some kind of alien is taking over the infected brain.
In fact, it is possible that what looks like mind control may be the result of hypersecretion of dopamine, a neuromediator that modifies the activity of brain cells.
During its evolution, the parasite has acquired the ability to produce an enzyme that increases the production of dopamine by the organism that hosts it.
This excess could induce a cerebral dysfunction with the consequence that the reactions of flight and approach are triggered respectively by the odors of predators or sexual partners.
Recently, a study found that the loss of fear in rodents infected with the parasite was not explicitly targeted to felids.
Uninfected mice ran away from danger, but infected ones approached the same environment casually and with curiosity.
This loss of fear was more pronounced the more parasite-infected foci were present in the cerebral cortex of the animals.
In humans, when infected by the parasite, studies have shown behavioral disorders such as loss of self-confidence, feelings of insecurity, or obsessive tendencies
The intestinal microbiota and our defense systems
The digestive tract mobilizes a large part of our immune system’s strength. Indeed, the quality of exchanges between the intestinal microbiota and the immune system cells plays an essential role in the maintenance of good health.
Immune cells are numerous in the intestinal wall. However, two types of cells have privileged access to the intestinal lumen:
- Paneth cells, which are present in the epithelium and secrete antibacterial substances
- Dendritic cells, which are located under the epithelium and emit extensions to the intestinal lumen.
The quality of mucus is essential to the immunological balance of our body
Mucus, lining the outer side of the intestines, is a significant physical barrier that reduces direct contact between the intestinal flora and the epithelial cells.
In some research, mice genetically incapable of secreting normal intestinal mucus have been observed to spontaneously develop severe inflammatory colitis and complications related to the passage of microbes into the bloodstream.
In germ-free mice, the mucus layer is much thinner, despite a retained number of caliciform cells responsible for mucus secretion. This demonstrates the critical role of substances produced by the microbiota in regulating mucus secretion.
Mucus expresses blood groups in 85% of individuals and these groups contribute to the selection of bacteria in the microbiota.
This is because mucus contains many antimicrobial chemicals secreted by Paneth cells.
Immunological monitoring of the intestinal microbiota also benefits from “feelers.” These are defined as dendritic cells that penetrate the intestinal lumen and ensure the absence of dangerous or unknown microbial elements that would require rapid intervention.
In a healthy mucosa, most of the active forces of immunity operate on the intestinal side of the epithelium.
When the mucus quality is reduced, due to damage to the intestinal wall or severe dysbiosis, intestinal microbes can approach the epithelial cells.
Then, microbes can insert themselves between these epithelial cells and enter the body, shifting the activity of the immune defenses to the depth of the intestinal wall with resulting health consequences.
This phenomenon is called “bacterial translocation” and is the cause of severe diseases.
The intestinal microbiota plays an essential role in immunity development and regulation
Numerous studies have shown the relationship between intestinal microbiota and immunity. The production of intestinal bacterial signals plays an essential role in the immune system’s balance.
These bacteria allow the immune system to develop properly and exercise its unwavering protection. These bacteria demonstrate the tolerance necessary for collaboration with the intestinal microbiota.
In these studies with germ-free mice, both innate and adaptive immunity’s defense capabilities against viral infections are diminished, hence the importance of the intestinal microbiota for the maturation and efficiency of our immune system.
The germs of the microbiota are also involved in the tolerance mechanisms that prevent the immune system from attacking useful symbiotic germs or the body’s own tissues.
The breakdown in tolerance is the cause of autoimmune diseases and inflammation.
The intestinal microbiota is a weight regulator
Our energy balance measures the balance between the energy intake from our food and our body’s energy expenditure.
This ensures the proper functioning of the vital functions of the human body, such as breathing, thermal regulation, or brain function.
In addition, it also allows the body to maintain the physical activities that we demand of it professionally and for leisure.
This balance between food and energy expenditure is very delicate and valuable in maintaining a normal weight.
The microbiota produces high-energy fatty acids through its ability to ferment undigested fibers. Approximately 35% of the energy provided by the diet is thus due to the action of the microbiota on the undigested substrates reaching the intestine.
In addition, certain short-chain fatty acids produced by fermentation are likely to modify the functioning of the satiety centers, regulating energy intake.
By acting as hormones on the receptors of the enteroendocrine cells, they can also modify fat storage in the organism and direct the major food processing pathways.
Changes in the production of short-chain fatty acids induced by a change in diet or alterations in the composition of the intestinal microbiota are likely:
- To have a significant impact on our energy balance
- To induce weight gain and metabolic complications bad for our quality of life and health.
What is the role of the intestinal microbiota in the maintenance of health? Conclusion
This article explains the role of the intestinal microbiota in the maintenance of good health.
A healthy microbiota is rich in a wide variety of microorganisms. Its composition is stable, and its most essential functions are redundantly dependent on several families of microorganisms.
Having abnormalities of the microbiota can lead to consequences, potentially severe, in three areas closely related to the intestinal microorganisms, namely:
- brain activity
- energy balance, and
The billions of bacteria that live in our intestines have significant roles in maintaining a healthy body.
Therefore, it is essential to take care of our intestinal microbiota because an imbalance of the latter can lead to severe diseases and degradation of our body.
Adopting a healthy lifestyle associated with a balanced and varied diet while practicing regular physical activity would help keep our microbiota in good shape!
Tip: For tips on what to eat to make your intestinal microbiota happier, read our other post What foods restore the intestinal flora? Simple & natural solutions