Microbiom uman

Microbiom uman, o gamă completă de microorganisme (microbiota) care trăiesc și la oameni și, mai precis, colecția de genomi microbieni care contribuie la portretul genetic mai larg sau metagenomul unui om. Genomii care constituie microbiomul uman reprezintă o gamă de microorganisme remarcabil de diverse, care include bacterii, arhaea (organisme unicelulare primitive), ciuperci și chiar unele protozoare și viruși fără viață. Bacteriile sunt de departe cei mai numeroși membri ai microbiomului uman: populația bacteriană singură este estimată între 75 de trilioane și 200 de trilioane de organisme individuale, în timp ce întregul corp uman este format din aproximativ 50 de trilioane până la 100 de trilioane de celule somatice (corporale). Abunda abundență microbiană sugerează că organismul uman este de fapt un „supraorganism,”O colecție de celule și gene umane și microbiene și astfel un amestec de trăsături umane și microbiene.

Descoperirea microbiomului uman

Primele dovezi științifice conform cărora microorganismele fac parte din sistemul uman normal au apărut la mijlocul anilor 1880, când pediatrul austriac Theodor Escherich a observat un tip de bacterii (numit mai târziu Escherichia coli) în flora intestinală a copiilor sănătoși și a copiilor afectați de boala diareică. În anii care au urmat, oamenii de știință au descris o serie de alte microorganisme izolate din corpul uman, inclusiv în 1898 specia Veillonella parvula, membră bacteriană a florei orale, digestive, urinare și respiratorii superioare, iar în 1900 bifidobacterii, membri ai flora intestinală. De-a lungul secolului XX, o serie de alte microorganisme au fost izolate de pasajele nazale, cavitățile orale, pielea, tractul gastro-intestinal și tractul urogenital și au fost caracterizate ca parte a microbiotei umane.Deși acest grup de organisme a fost conceptualizat în diferite moduri de la descoperirea sa, conceptul microbiomului uman și, prin urmare, studiul intensiv al acestuia, a fost dezvoltat în primul rând în prima decadă a secolului XXI.

Knowledge of the human microbiome expanded appreciably after 2007, the year the Human Microbiome Project (HMP)—a five-year-long international effort to characterize the microbial communities found in the human body and to identify each microorganism’s role in health and disease—was launched. The project capitalized on the decreasing cost of whole genome sequencing technology, which allows organisms to be identified from samples without the need for culturing them in the laboratory; the technology also facilitates the process of comparing DNA sequences of microorganisms isolated from different parts of the human body and from different people. In the first three years of the project, scientists discovered new members of the human microbiota and characterized nearly 200 different bacterial member species.

Microbial diversity

By some estimates, the human microbiota may consist of a total of 900 or 1,000 different species of microorganisms, making for an extraordinarily diverse collection of microbial genomes. This diversity manifests in differences in microbial composition not only from one human to the next but also between matching body parts, such as the right and left hands, of the same individual. For example, as one study has shown, a typical palm surface of the hand can harbour more than 150 different bacterial species, only 17 percent of which are common to both hands of the same person and only 13 percent of which are shared by different persons.

The human gut is another site characterized by a high degree of microbiome diversity and abundance. In a study of 124 European individuals, researchers isolated some 3.3 million microbial genes. Many of these genes represented frequently occurring bacterial gut species, at least 160 of which were believed to inhabit each person’s gut. The identification of such frequently occurring species in populations is fundamental to defining so-called common bacterial cores, which enable scientists to explore the interface of the human microbiome with factors such as diet, culture, and genotype (genetic makeup).

The role of the human microbiota

Most members of the human microbiota benefit humans by providing them with traits that they would not otherwise possess. Some microorganisms found in the human gut, for instance, obtain nutrients from ingested food in return for assisting with the breakdown of food or preventing the colonization of the gut by harmful bacteria. There are, however, many microorganisms in the human microbiota that are closely related to pathogenic (disease-causing) organisms or are themselves capable of becoming pathogenic. Examples include bacterial species of the genera Staphylococcus, Streptococcus, Enterococcus, Klebsiella, Enterobacter, and Neisseria.

Clostridium difficile infectionserves as a useful example for illustrating the significance of the relationship between the human microbiome and health and disease. C. difficile infection, which is characterized by severe recurrent diarrhea, abdominal cramping, and nausea, occurs most often in persons who receive a course of antibiotics while in a hospital. Antibiotics kill or inhibit the reproduction of pathogenic bacteria and in the process cause dramatic changes in normal human microbial communities, such that previously established colonies may be overtaken by colonies of different and potentially pathogenic species. In the case of C. difficile, researchers have discovered that infection can be treated effectively through fecal, or stool, transplantation, in which fecal material from a healthy person is transferred to the patient, thereby restoring populations of beneficial gut microbiota.

Scientists studying obesity have detected an increased abundance of Prevotella and Firmicutes bacteria and of methanogenic (methane-producing) archaea in obese individuals relative to normal-weight persons and persons who have undergone gastric bypass surgery. Scientists suspect that these microorganisms are more efficient at harvesting carbohydrates from food than are the types of microorganisms that dominate the gut flora of normal-weight individuals. The extra nutrients are then stored in the body as fat.

Ongoing study of the human microbiome is expected to continue to shed light on fundamental aspects of human physiology and particularly human nutrition. Improved understanding of nutritional requirements could lead to changes in dietary recommendations and in food production. In addition, information about the human microbiome could lead to the development of new diagnostic techniques and treatments for a variety of human diseases, as well as to the development of industrial products based on substances (e.g., enzymes) that are produced by members of the human microbiota.