Although the presence of a large number and variety of microorganisms in very diverse locations in the human anatomy is well known, the role played by these prokaryotic, bacterial, eukaryotic or viral communities in health-disease processes is relatively recent. Currently we know that coexistence and attunement with these microorganisms, which we call together the microbiota, is not only not pathological, but it is essential for normal physiological functioning.
It could not be otherwise, since the evolutionary development of biosystems has taken place in specific microbiotic contexts, without which they would probably be meaningless. Indeed, the highest rates of colonization occur in those locations that constitute anatomical and functional barriers between the external and internal environments, such as the digestive tract or the integument.. In these locations, evolutionarily the creation and maintenance of a tolerogenic immunological environment has been favored, that is, tending to the emission of low inflammatory or “tolerant” responses towards antigens interpreted as foreign, such as food antigens or those from the clothing or cosmetics that interact with our skin. In its role as delimiter and manager of one’s own and that of others, the immune system plays a prominent role in the composition and interpretation of the posthumanist symphony that regulates systemic homeostasis.
Despite the remarkable existence of large-scale sequencing projects such as The Human Microbiome Project or The Earth Microbiome Project, there is growing evidence that the composition of the microbiota or the microbiome is not sufficient to construct a coherent, comprehensive and vivid ontoepistemic narrative. . Similar to what happened at the time with the human genome and epigenome in phenotypic genesis and maintenance, academic attention is progressively focusing on the complex and multidirectional interaction processes between microorganisms, host and environment.It has been shown that it is in this constant dialogue, which clearly transcends the human dimension, where health-disease lies. So much so that, in studies of discordant homozygous twins for the phenotypic expression of a disease, significant differences have been observed in the composition of the microbiota, from which it is deduced that colonization and vital exposure to bioactive factors – although probably influenced by genetic factors – would be the determining variables when explaining the microbiotic composition of a person at a given moment in his life. Therefore, the microbiome should not be understood as something static and characteristic of the host organism, but rather the opposite:
DYSBIOSIS SEEM TO PLAY A KEY ROLE IN THE GENESIS AND MAINTENANCE OF SOME PATHOGENIC PROCESSES
The microbiome, “the great forgotten organ”?
The medical interest in the study of the microbiome is enormous and growing, since what some authors call “the great forgotten organ” has been related to a large number of pathologies that are very prevalent in contemporary western populations. Specifically, dysbiosis (abnormalities in the relative composition of the microbiota that result in imbalances between the different species or families that compose it) seem to play a key role in the genesis and maintenance of autoimmune and / or inflammatory pathogenic processes, that affect practically all human organs and systems. Inflammatory bowel diseases, psoriasis, diabetes, multiple sclerosis, or neurodevelopmental disorders are just a few examples, as well as – logically – the susceptibility and pathogenicity of infections by specific digestive pathogens, especially eukaryotes (eg giardiosis, amebiosis or trichomonas vaginosis). Not only do specific nosological entities present differentiated patterns of microbiological composition, but physiological conditions such as obesity or malnutrition have also been correlated with the relatively increased or decreased presence of certain microbiological families. In addition to the differences resulting from phylogenetic and ontogenetic exposure to different potentially pathogenic microorganisms, it has been shown that dietary changes, the use of antibiotics and other pharmacological agents, surgery or allogeneic transplants can alter the microbiota and / or its interactions with the environment in a clinically relevant way.
DIETARY CHANGES, THE USE OF ANTIBIOTICS AND OTHER PHARMACOLOGICAL AGENTS, SURGERY OR ALLOGENEIC TRANSPLANTS CAN ALTER THE MICROBIOTA AND / OR ITS INTERACTIONS WITH THE ENVIRONMENT IN A CLINICALLY RELEVANT WAY
Main components of the human microbiota
Significant differences have been observed in the compositions of the commensal flora located in the different anatomophysiological barriers, such as the digestive tract, the skin or the vagina; However, in the same location and in conditions of apparent health and environmental constancy (that is, without sudden changes in the factors that could alter the microbiome, such as diet or taking antibiotic or immunomodulatory medication), a Relative stability of the composition of the commensal flora in the subjects throughout most of their lives. Broadly speaking, and in western or westernized populations (as we will explain in greater detail in the next paragraph), the bacterial families called Bacteroidetes and Firmicutes, which each include dozens of genetically related species of bacterial microorganisms, are considered the main components of the human microbiota. At the individual species level, some popularly known names corresponding to specimens of the human commensal flora at the gastrointestinal level are Helicobacter pylori , Escherichia coli , Clostridium difficile or Lactobacillus spp.. As the reader will have noticed, these microorganisms are also known as pathogens or as species related to the etiopathogenesis of infections, which occurs mainly under specific conditions of coexistence with other microorganisms, environmental factors and factors related to the configuration of the immune system – innate or adaptive – of the host, all of them geotemporally specific.
The geotemporal variation gradient of the microbiotic composition, both at community and individual level, constitutes a complex question far from being elucidated by the scientific community. The sociopolitical biases of biomedical research carry the risk of a fictitious simplification of the question, since most of the studies have been carried out in subjects residing in Western countries, and therefore subject to certain dietary patterns (eg high consumption of animal protein from red meat, low consumption of vegetable fibers, diets high in fats and sugars) and to the treatments typical of Western medicine. However, as can be seen in the figure above (taken from the article by Clemente, Ursell, Parfrey et al.  ), it is known thatthe composition of the microbiota stabilizes around the first two years of life. The gastrointestinal tract is currently considered in uteroas practically sterile (despite having found the presence of microorganisms in small quantities in the amniotic fluid from sacks of healthy born babies). During the first months of life, the microbiota shows significant intersubject differences depending above all on the route of delivery (vaginal, cesarean section), diet and medical treatments received during this first period of life. As a result, the highest rates of colonization and diversification of the commensal flora take place during these first two years, after which under normal conditions there is a progressive stabilization and assimilation of the flora to the normality of the reference population of the subject. In the same way,