Human microbiota: definition, composition, and function

It has been known for almost a century that humans harbor a microbial ecosystem, known as human microbiota, remarkably dense and diverse, made up of a number of viruses and cells much higher than those of the human body, and that accounts for one to three percent of body weight. All the genes encoded by the human body’s microbial ecosystem, which are about 1,000 times more numerous than those of our genome, make up the human microbiome. Microorganisms colonize all the surfaces of the body that are exposed to the environment. Indeed, distinct microbial communities are found on the skin, in the vagina, in the respiratory tract, and along the whole intestinal tract, from the mouth up to rectum, the last part of the intestine.

Contents

• Composition
• Function of the human microbiota
• Human Microbiome Project
• Effect of antibiotics
• References

Composition

It is composed of organisms from all taxa.

• Bacteria, at least 100 trillion (10¹⁴) cells, a number ten times greater than that of the human body. They are found in very high concentration in the intestinal tract, up to 10¹²-10¹⁴/gram of tissue, where they form one of the most densely populated microbial habitats on Earth. In the gut, bacteria mainly belong to the Firmicutes, Bacteroidetes and Actinobacteria phyla. Fusobacteria (oropharynx), Tenericutes, Proteobacteria, and Verrucomicrobia are other phyla present in our body.
  Note: Bacterial communities in a given body region resemble themselves much more across individuals than those from different body regions of the same individual; for example, bacterial communities of the upper respiratory tract are much more similar across individuals than those of the skin or intestine of the same individual.
• Viruses, by far the most numerous organisms, about quadrillion units. The genomes of all the viruses harbored in the human body make up the human virome. In the past, viruses and eukaryotes have been studied focusing on pathogenic microorganisms, but in recent years the attention has also shifted on many non-pathogenic members of these groups. And many of the viral gene sequences found are new, which suggests that there is still much to learn about the human virome. Finally, just like for bacteria, there is considerable interpersonal variability.
• Archaebacteria, primarily those belonging to the order Methanobacteriales, with Methanobrevibacter smithii predominant in the human gut (up to 10 percent of all anaerobes).
• Eukaryotes, and the parasites of the genera Giardia and Entamoeba have probably been among the first to be identified. But there is also a great abundance and diversity of fungal species, belonging to genera such as Candida, Penicillium, Aspergillus, Hemispora, Fusarium, Geotrichum, Hormodendrum, Cryptococcus, Saccharomyces, and Blastocystis.

Based on the relationships with the human host, microorganisms may be classified as commensals or pathogens.

• Commensals cause no harm to the host, with which they establish a symbiotic relationship that generally brings benefits to both.
• On the contrary, pathogens are able to cause diseases, but fortunately represent a small percentage of the human microbiota. These microorganisms establish a symbiosis with the human host and benefit from it at the expense of the host. They can cause disease:

if they move from their niche, such as the intestine, into another one where they do not usually reside, such as the vagina or bladder, as in the case of Candida albicans, normally present in the intestine, but in very small quantities;
in patients with impaired immunological defenses, such as after an immunosuppressive therapy.

Function of the human microbiota

Sometimes referred to as “the forgotten organ”, human microbiota, mainly with its intestinal bacterial members, plays many important functions that can lead to nutritional, immunological, and developmental benefits, but can also cause diseases. Here are some examples.

• It is involved in the development of the gastrointestinal system of the newborn, as shown by experiments carried out on germ-free animals in which, for example, the thickness of the intestinal mucosa is thinner than that of colonized animals, therefore more easily subject to rupture.
• It contributes to energy harvest from nutrients, due to its ability to ferment indigestible carbohydrates, promote the absorption of monosaccharides and the storage of the derived energy. This has probably been a very strong evolutionary force that has played a major role in favor of the fact that these bacteria became our symbionts.
• It contributes to the maintenance of the acidic pH of the skin and in the colon.
• It is involved in the metabolism of xenobiotics and several polyphenols.
• It improves water and mineral absorption in the colon.
• It increases the speed of intestinal transit, slower in germ-free animals.
• It has an important role in resistance to colonization by pathogens, primarily in the vagina and gut.
• It is involved in the biosynthesis of isoprenoids and vitamins through the methylerythritol phosphate pathway.
• It stimulates angiogenesis.
• In the intestinal tract, it interacts with the immune system, providing signals for promoting the maturation of immune cells and the normal development of immune functions. And this is perhaps the most important effect of the symbiosis between the human host and microorganisms. Experiments carried out on germ-free animals have shown, for example, that:

macrophages, the cells that engulf pathogens and then present their antigens to the immune system, are found in much smaller amounts than those present in the colonized intestine, and if placed in the presence of bacteria they fail to find and therefore engulf them, unlike macrophages extracted from a colonized intestine;
there is not the chronic non-specific inflammation, present in the normal intestine as a result of the presence of bacteria (and of what we eat).

• Changes in its composition can contribute to the development of obesity and metabolic syndrome.
• It protects against the development of type I diabetes.
• Many diseases, both in children and adults, such as stomach cancer, lymphoma of mucosa-associated lymphoid tissue, necrotizing enterocolitis, an important cause of morbidity and mortality in premature babies, or chronic intestinal diseases, are, and others seem to be, related to the gut microbiota.

In conclusion, it seems very likely that the human body represents a superorganism, result of years of evolution and made up of human cells, and the resulting metabolic and physiological capacities, as well as an additional organ, the microbiota.

Human Microbiome Project

The bacterial component of the human microbiota is the subject of most studies including a large-scale project started in 2008 called “Human Microbiome Project”, whose aim is to characterize the microbiome associated with multiple body sites, such as the skin, mouth, nose, vagina and intestine, in 242 healthy adults.
These studies have shown a great variability in the composition of the human microbiota; for example, twins share less than 50 percent of their bacterial taxa at the species level, and an even smaller percentage of viruses. The factors that shape the composition of bacterial communities begin to be understood: for example, the genetic characteristics of the host play an important, although this is not true for the viral community. And metagenomic studies have shown that, despite the great interpersonal variability in microbial community composition, there is a core of shared genes encoding signaling and metabolic pathways. It appears namely that the assembly and the structure of the microbial community does not occur according to the species but the more functional set of genes. Therefore, disease states of these communities might be better identified by atypical distribution of functional classes of genes.

Effect of antibiotics

The microbiota in healthy adult humans is generally stable over time. However, its composition can be altered by factors such as dietary changes, urbanization, travel, and especially the use of broad-spectrum antibiotics. Here are some examples of the effect of antibiotic treatments.

• There is a long-term reduction in microbial diversity.
• The taxa affected vary from individual to individual, even up to a third of the taxa.
• Several taxa do not recover even after 6 months from treatment.
• Once the bacterial communities have reshaped, a reduced resistance to colonization occurs. This allows foreign and/or pathogen bacteria, able to grow more than the commensals, to cause permanent changes in human microbiota structure, as well as acute diseases, such as the dangerous pseudomembranous colitis, and chronic diseases, as it is suspected for asthma following the use and abuse of antibiotics in childhood. Moreover, their repeated use has been suggested to increase the pool of antibiotic-resistance genes in our microbiome. In support of this hypothesis, a decrease in the number of antibiotic-resistant pathogens has been observed in some European countries following the reduction in the number of antibiotics prescribed.

Finally, you must not underestimate the fact that the intestinal microflora is involved in many chemical transformations, and its alteration could be implicated in the development of cancer and obesity. However, regarding use of antibiotics, you should be underlined that if western population has a life expectancy higher than in the past is also because you do not die of infectious diseases!

References

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