Have you ever stopped to think about the coexistence you have with other organisms? Have you thought about your dog, cat, or pet? Think a little more about something smaller, invisible to the eyes... If the latter reminds you of microorganisms, excellent! And if not, don't worry, we will talk to you about a group of them in this article.

But first, some definitions...

Microorganism: Is an organism that is microscopic and can be bacteria, fungi, viruses, archaea or protozoa.

Microbiome: Is the set of these organisms coexisting with the largest numbers found in the small and large intestines but also throughout the body. The microbiome consists of microbes that are both helpful and potentially harmful. Most are symbiotic (where both the human body and microbiota benefit) and some, in smaller numbers, are pathogenic (promoting disease).

Metagenomics: A method that allows the study of the structure and function of nucleotide sequences in organisms, generally microbes, residing in a specific environment.

Dysbiosis: Alteration in the composition and/or functions of microorganisms inhabiting a specific area of the organism. For this article, we will refer to the bacteria residing in the interior of the intestines.

Specifically, we will talk about bacteria and their relationship with us. To do this, let's go back to the maternal womb where the presence of bacteria in the fetal environment indicates that our intestinal microbiota interacts with the host (the fetus). This happens thanks to bacterial extracellular vesicles (Fig.1) present in the fetal environment, namely, the amniotic fluid.

These vesicles play a role in preparing the prenatal immune system for subsequent intestinal colonization at birth. This colonization will have an effect on health during the neonatal period, which encompasses the first four weeks of the baby's life until adulthood. When we were small, the plasticity and adaptation of our bacteria conferred advantages, reducing the risk of developing diseases or, on the other hand, causing an imbalance related to diseases that can develop at any stage of our life. These changes in our microbiome occur due to the interactions we have with the environment around us and our physical and dietary habits.

Fig. 1 Extracellular vesicles of amniotic fluid. image taken from https://microbiomejournal.biomedcentral.com/articles/10.1186/s40168-023-01694-9/figures/1

So, in addition to mom's home remedies, it's the bacterial colonies residing in our intestines that continue the maturation and modification of the immune system. It's not just about protecting us from pathogens; they also regulate endocrine functions, neuronal signaling, energy, and the production of secondary metabolites, the functions of which are still being elucidated.

Currently, our gastrointestinal microbiota is considered an organ because, as mentioned before, they play a crucial role in the development, evolution, and degeneration of many systems in the human body.

As a curious fact, the simple fact of being born vaginally or through a cesarean section already modifies the type and quantity of microorganisms that will be with you in the first days of life, and although they will change as you grow, they will be crucial to your health. It is known that babies born through normal delivery have more protection against diseases than those born via cesarean section, as the latter only receive microbiota from the skin, which is not as beneficial or diverse as that from the vagina.

Well, let's move on to how beneficial colonies are maintained; one of the answers is the effect of nutrition.

For intestinal health, it is essential to consume:

Dietary fiber, which is composed of fruits, vegetables, whole grains, and legumes. Their consumption determines the ecology, diversity, and function of intestinal bacteria.

Dietary protein, which can be of plant origin, such as nuts, soy, legumes, and cereals, or of animal origin, including meats, fish, poultry, eggs, and dairy products. These serve as a nitrogen resource for the bacteria residing in the colon, allowing them to metabolize into products beneficial to the intestinal microbiota.

If, on the contrary, our diet is predominantly high in fats, we will create an adverse environment for our bacteria and intestinal absorption.

Although diet is an important factor in maintaining our bacteria, there are other causes such as the use of antibiotics, exposure to pollutants, age, gender, interaction with other bacteria, and lifestyle, all of which can alter their composition and function.

And amidst all this, how many types of bacteria do we have?

Our intestinal microbiome hosts approximately 500 to 1000 species of intestinal microbes. Through metagenomic studies, the predominant microbial communities in the intestinal microbiome have been deciphered, belonging to the Eubacterial phyla, including Firmicutes, Bacteroidetes, Proteobacteria, Fusobacteria, Cyanobacteria, Verrucomicrobia, and the Archaeobacterial phylum Euryarchaeota. These organisms play a role in digestion, production of metabolites, and molecules that are important in cellular signaling. They control the cellular health of the host, have a role in immunity, and guess what? They also play a role in neuro-psychiatric behaviors.

The Brain and the Intestinal Microbiome

Personality, constituted by a series of traits and qualities shaped by social interaction, makes us unique and allows us to differentiate ourselves from others. It turns out that this external environment is not the only one influencing our behavior; the study of the microbiome affects the physiology of our body and even our behavior.

Dysbiosis of the intestinal microbiota may be related to neurological and psychiatric disorders. Studies have shown that the microbiota can trigger stress, anxiety, and depression responses, as well as changes in communication and behavior. This is possible because microbial communities interact via neural, endocrine, and immune pathways, as organisms can produce neuroactive metabolites that can alter levels of certain neurotransmitters. These neurotransmitters send excitatory or inhibitory signals to our neurons.

Here are some bacterial genera and their associations with certain behaviors or psychiatric conditions:

- Akkermansia in children with autism

- Allistipes in children with autism and adults with depression

- Bacteroides in children with autism and adults with constant negativity and psychosis

- Bifidobacterium in children with autism and adults with depression and negativity

- Blautia in infants with autism, children with autism, and adults with schizophrenia

Based on existing results, therapies have been developed through the administration of probiotics, demonstrating their ability to modulate patients' brains and improve their initial neurological condition.

Although there are already several studies on the intestinal microbiome and its relationship with other parts or systems of the body, many questions remain unanswered. This provides a glimpse into what our bacterial friends do for us. It is important to continue delving into metabolites and their role in health and diseases, as well as the development of probiotics and biomarkers.