The authors discuss the theory of human superorganism and its microbiota (microbiome), whose mutualistic interactions is realized within the microbiota – gut – brain axis that includes endocrine, immune and neurohumoral pathways. The newest concepts of microbiome enterotypes and core microbiota are presented, which are important for understanding of the role of symbiotic microorganisms in human vital activities, for explanation of pathophysiology of many chronic human diseases (beyond gastrointestinal disorders), as well as for the search of effective therapeutic targets. As highly promising are considered the functional approaches to studies of microbiota that allowed to formulate the concept of phylometabolic (phylofunctional) core. This is a series of evolutionally stable microorganisms responsible for majority of the main microbiome functions, such as fermentation of polysaccharides (glycans), production of short-chain fatty acids (butyrate, propionate, acetate), hydrogen utilization, production of lactate, metabolism of aminoacids, bile acids, choline, production of vitamins and some biologically active substances – anti-inflammatory, anti-microbial, immunostimulatory. The authors are first to describe the main functional groups of microorganisms of gut microbiota phylometabolic core, providing key metabolic functions, as well as the leading characteristics of the phylometabolic core as such. The perspectives of modification of composition and functions of phylometabolic microbiota core are discussed based on metabiotics as a virtually new class of therapeutic agents. A hypothesis has been proposed that the ratios between main components of the key gut microbiota may reflect fundamental processed related to a mutualistic interactions between microbiota and human body, as well as they may serve as effective biological markers of dysbiotic states determining the development of various pathologic conditions. For example, the ratio between Bacteroides spp. and butyrate producing bacteria that indirectly indicates total numbers of microbial genes can be used both for assessment of chronic inflammation of various localization (starting from inflammatory bowel disease to fat tissue inflammation related to metabolic syndrome), and for control of treatment efficacy.