microbiota and Man: an intense relationship between microorganisms and humans
The interaction between humans and microorganisms is inevitable and many of them as soon as we leave the protective womb in which there are sterile conditions, establishing their residence in or on the body. Microorganisms are present in all districts of our body, except the central nervous system (CNS), blood and internal organs and tissues, and the increased concentration of metabolic activity and therefore they can be found in the last portion of the intestine. The set of microbial populations in the human body is called microbiota (Tannok, 1995) and is equivalent to that of normal microbial flora. The composition of microbial communities that live in our body is affected by genotype, sex, age, state of immune maturation and environmental factors and is therefore highly variable in human populations. For example, in the skin permanently resident Gram-positive bacteria such as Staphylococcus epidermidis and Micrococcus species of the genus that have adapted to particular environmental conditions, slightly acidic pH, presence of antimicrobial secretions and use the sebum that is rich in organic acids, lipids and amino acids. The oral cavity contains a complex microbiota, about 500 species, mostly bacteria, but even protozoa (Entamoeba gingivalis, Trichomonas tenax) and fungi (Candida albicans), which consists of facultative anaerobes and required, each species occupies a particular ecological niche in relation to their environmental requirements. For example, Streptococcus mutans and Streptococcus salivarius species which are facultative anaerobes, growing respectively on the enamel of the teeth and tongue, while species of the genus Bacteroides, as the obligate anaerobes have adapted to grow in gum pockets where there are anaerobic conditions .
Before focusing on microbiota of the intestinal tract, we have to make some considerations: 1) the prokaryotic cells in our body at least a factor of 10 than those of eukaryotic 2) the weight of the entire mass of resident microbes in our body is about a kilo, or more than some important organs like the heart or kidneys, and then we could talk about a real microbial organisms in more than 3) genetic analysis carried out by an international consortium METAHIT (Metagenomics of the Human Intestinal Tract) has shown that the microbiome, or the complex genes belonging to the microbiota, has 150 times more genes of the human species that hosts (and Dusko METAHIT, 2010). These considerations lead us to say that from the biological point of view, we humans should not consider the individual, but rather a highly diverse community and co-developed by members belonging to the domains of eukaryotes, bacteria and archaea.
microbiota of the intestinal tract
The upper intestine is sparsely populated by microbes, but from the ileum microbial concentration grows to 1011-1012 colony forming units per gram (cfu / g) in the colon. Up to 1000 microbial species, but the number is certainly underestimated, are present in the large intestine and it is believed that 30 to 50% of the content of the human colon is made up of bacteria. The concentration (in terms of density microbial population) and composition (in terms of genres and types of species present) is due to several environmental factors in the various districts of the intestinal tract. Since environmental conditions vary along the intestinal tract, also changes the composition of the microbiota. The intense flow of content in the first section does not allow a large accumulation of microorganisms and pancreatic secretions and liver limit the accumulation of them. At the bottom of the flow of the intestinal tract contents (digested food) becomes slower and this favors the large increase in the microbial community in terms of cfu / g. The diet of the individual can influence the composition of the community resident microbes, eg bifidobacteria are more prevalent in vegetarian subjects. Due to the anaerobic conditions of the lower intestinal tract, bacteria are predominant obligate anaerobes than optional. There may be significant differences in the composition depending on whether you look at the luminal side or the mucosal intestine: a mucosal level can be microaerobic conditions (oxygen coming from the tissues) and a greater concentration of antimicrobial substances (lactoferrin, and defensins other antimicrobial peptides). Consequently, there may be variability depending on species or strains of the consignment. The gut microbial species may be transient or permanent. Most species are commensals, some are potentially pathogenic, others play a beneficial role for the health of the host. The strains with beneficial properties recognized, belonging to the genera Bifidobacterium and Lactobacillus, in this case, the relationship you establish with these intestinal microbiota can be described as mutualistic symbiosis with benefits for both partners of the report. However, the function of most members of the intestinal microbial community is unknown why it is often difficult to assign a role to each species. Some potential pathogens (Candida, Clostridium) at low population density, could play a role beneficial contribution to the maturation of the immune system. The intestinal microbiota is unique to each individual and is the result of the interaction between the human host and an environment populated by microorganisms from birth and throughout life. Microbial communities have co-evolved with their human hosts or animals. Our first encounter with microbes is in the birth canal and the first colonization of the intestinal tract (initially sterile) is due to maternal intestinal and vaginal microbiota. Before birth the fetus, including the intestine is sterile, the birth interrupts this sterility: the microbial colonization begins immediately, natural childbirth exposes the child to the flora microbial mother (intestinal and vaginal) and includes genres such as Bacteroides, Lactobacillus, and enteric bacteria that are the initial source of bacteria. The relationships between humans, the ingestion of food and all the contacts define and stabilize the environmental microbial community persisted. Microbial species are periodically introduced transitional dynamics of an association with the result that lasts a lifetime. In the age of the host, the first microbes are facultative anaerobes, however, obligate anaerobes (Bacteroides, Bifidobacteria) appear a few days after birth. The facultative anaerobes such as enterobacteria, they reduce the redox potential of the intestine by promoting colonization by anaerobes (Isolauri et al, 2004).
ROLE OF NUTRITIONAL-METABOLIC microbiota INTESTINAL
The intestinal microbiota in the colon plays a major metabolic activity for example is involved in the fermentation of undigested oligosaccharides of exogenous origin (fibers, cellulose, etc.). Endogenous or (mucopolysaccharides, mucin). Fermentation of various types of oligosaccharides is of benefit to the host because it provides additional sources of energy as short-chain fatty acids. For example, butyric acid is a source of energy for intestinal epithelial cells and is important for the health of the colonic mucosa. Another aspect is the production vitamins (B complex and K) and metabolism of bile acids with consequent reduction in the available pool of bile acids for the synthesis of cholesterol. In addition, the microbiota improve digestion and absorption of some nutrients and increase the surface area.
microbiota and immune system
The microbiota represent a stimulus for the immune system (antigen challenge) and cause the maturation of the gut associated lymphoid tissue. At birth, our immune system is immature and is spread through exposure to microbes: the adjoining gut lymphoid tissue (eg Peyer's patches) and the production of IgA, an increase due to the presence of intestinal microbiota. Therefore, the microbiota are important for the development of constitutive and specific host defenses at mucosal. Preliminary data obtained in studies on human hosts indicates a role for bifidobacteria in the maturation of the immune system and allergic states. Colonization with Bacteroides species of the genus is associated with the maturation of the humoral immune system.
physiological inflammation: This phenomenon is due to massive stimulation of the mucosal immune system by luminal antigens Costituto mainly by bacteria of the intestinal microflora and their components. The presence of bacteria in the intestine promotes the emergence of populations of cells that maintain the epithelium in a state of physiological inflammation, to facilitate the generation of a rapid defense response against invading pathogens.
immuno-modulatory role: In some allergic diseases of childhood have observed lower levels of bifidobacteria and higher levels of species of the genus Clostridium than in healthy subjects. Other studies have reported a predominance of Bifidobacterium adolescens in allergic children and in healthy children is the Bifidobacterium bifidum to prevail. These two species suggest a different profile of cytokines, in vitro studies it was shown that Bifidobacterium adolescens induces the secretion of TNF, IL-1, IL-6, IL-12 by macrophages, and Bifidobacterium bifidum stimulates the production of IL-10 by regulatory T lymphocytes (Isolauri et al, 2008).
ANTAGONISM WITH PATHOGENIC
The intestinal microbiota provide protection against pathogenic micro-organisms with a dual strategy: competition for nutrients and sites of pathogen attack and applying a barrier function through the production of bacteriocins (antimicrobial peptides, such acidophylina produced by L.acidophylus) acting antipatogena (Servin et al, 2004).
The importance of the beneficial role of the microbiota has been demonstrated using animals (especially birds, rodents and farm animals) Axenia or germ-free, or to be born and maintained under sterile conditions and thus of no microbiota. Animals Axenia need the addition of vitamin K in their diet have an increased susceptibility to infectious diseases, also the adjoining gut lymphoid tissue and consequently the humoral mucosal immunity is poorly developed compared to that of conventional animals. Normally co-exist peacefully with our microbiota, however, any factor that alters our ecosystem gastrointestinal (GI) can potentially lead to disease. For example, the antibiotic treatment, surgery, chemotherapy or concomitant chronic or degenerative diseases that weaken our immune system, can cause the invasion of the resident microbiota of our systemic areas (areas taboo, or sterile saline). Appendix perforation or accidental perforation of the intestine during surgery allows the invasion of bacteria in the peritoneal cavity that surrounds the GI tract. and from there enter the bloodstream.
Conclusions The complexity of the microbiota makes it difficult to assign a role to each microbial species, for example, that allows us to establish a cause-effect relationship between certain diseases and altered composition of the microbiota. Some researchers have used "gnotobioti" or bacterial species known implanted in germ-free animal. Obviously the study of community microbial as a whole - at least 1000 species-can not be studied in this way. There remain several open questions: for example, understanding the molecular mechanisms of the host-organism within the gut (communication between the microbiota and epithelial cells and the regulation of gastrointestinal endocrine cells), the relationships between different microbial species microbiota and the effects of these interactions with the host. Another objective of the research in this field is to identify and characterize the microbial species of beneficial microbiota as a source of new potential therapeutic use of probiotic species (Tuohy et al, 2003).
Bibliography
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Isolauri E, Salminen S, Ouwehand AC Microbial-gut Interactions in health and disease. Probiotics.
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Isolauri E, Kalliomäki M, Laitinen K, Salminen S. Modulation of the maturing gut barrier and microbiota: a novel target in allergic disease. 2008. Curr Pharm Des.14 (14) :1368-75.
Servin AL Antagonistic Activities of lactobacilli and bifidobacteria against microbial pathogens 2004. FEMS Microbe. 28: 405-440
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Tuohy, KM. Probert HM, Smejkal CW, Gibson GR. Using probiotics and prebiotics to Improve gut health. 2003. Drug Discovery Today. 8 (15), 692-700
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