Although the presence of a large number and variety of microorganisms in very diverse locations of the human anatomy is well known, it is relatively recent recognition of the role played by these communities of bacterial, eukaryotic or viral prokaryotic nature in the health-disease processes. Currently we know that the coexistence and harmony with these microorganisms, which we call together microbiota, not only is not pathological, but is essential for normal physiological functioning.
It could not be otherwise, since the evolutionary development of biosystems has taken place in concrete microbiotic contexts, without which they would probably be meaningless. In effect, the highest rates of colonization occur in those locations that constitute anatomofunctional barriers between external and internal means, such as the digestive tract or the tegument . In these locations, the creation and maintenance of a tolerogenic immunological environment has been favorably promoted, that is, aimed at the emission of non-inflammatory or “tolerant” responses to antigens interpreted as foreign, such as food antigens or those from the clothes or cosmetics that interact with our skin. In its role as delimiter and manager of one’s own and another’s, 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 are not sufficient to construct a coherent, integral and verbatim ontoepistemic narrative. . Similar to what happened at the time with the human genome and epigenome in the genesis and phenotypic maintenance, academic attention is being progressively focused on the complex and multidirectional interaction processes between microorganisms, host and environment , as it has been demonstrated that it is in this constant dialogue that 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 follows 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 time in his life. Therefore, the microbiome should not be understood as something static and characteristic of the host organism, but rather on the contrary: the dynamism and the responsiveness to environmental changes are essential characteristics in the conceptualization of this term.
THE 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 linked to a large number of well-known pathologies in contemporary Western populations. Specifically, dysbiosis (anomalies in the relative composition of the microbiota that result in imbalances between the different species or families that make it up) seem to play a key role in the genesis and maintenance of pathogenic processes with an autoimmune and / or inflammatory root, that affect practically all human organs and systems. Intestinal inflammatory diseases, psoriasis, diabetes, multiple sclerosis or neurodevelopmental disorders are just some examples , as well as – logically – the susceptibility and pathogenicity of infections by specific digestive pathogens, especially eukaryotes (eg giardiosis, amebiasis or vaginosis by trichomonas). Not only do specific nosological entities show different patterns of microbiotic composition, but physiological conditions such as obesity or malnutrition have also been correlated with relatively increased or decreased presence of certain microbiological families. In addition to 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 medium in a clinically relevant manner.
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 MANNER
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 (ie, without abrupt 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 subjects throughout most of their lives. Broadly speaking, and in western or westernized populations (as we will explain in more detail in the following paragraph), the bacterial families called Bacteroidetes and Firmicutes , which each include dozens of species of microorganisms of a genetically related bacterial nature, are considered main components of the human microbiota. At the level of individual species, 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 in 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 gradient of geotemporal variation of the microbiotic composition, both at a community and individual level, constitutes a complex issue far from being elucidated by the scientific community. The sociopolitical biases of biomedical research carry the risk of a fictional simplification of the issue, since most of the studies have been conducted in subjects resident in Western countries, and therefore subjected to certain dietary patterns (eg high consumption of animal protein from red meats, low consumption of vegetable fibers, diets high in fat and sugars) and the treatments 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 that the composition of the microbiota stabilizes around the first two years of life. At present, the gastrointestinal tract in utero is considered practically sterile (despite 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 presents significant intersubject differences depending mainly on the way of delivery (vaginal, cesarean), diet and medical treatments received during this first time 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 in 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, it is hypothesized that in the elderly the microbiota is significantly different than in adulthood as a consequence of the greater exposure to pathology and medical treatment in the last decades of human life.
 Clemente, JC; Ursell, LK; Parfrey, LW; & Knight, R. (2012). The Impact of the Gut Microbiota on Human Health: An Integrative View. Cell 148 , 1258-1270.