**1. Introduction**

The gastrointestinal tract is a major immunological organ that evolved to tolerate commensal and dietary antigens, yet retains the ability to mount a protective immune response to pathogens. The complex co-evolved community of the gut microbiota impacts the development of immunity and health of an individual. Although much of the gut microbiota is deemed non-culturable.,the advent of high-throughput sequencing techniques has greatly improved the ability to clarify gut microbiota composition and function. New technologies enable to include estimation of the gut microbiome diversity as well as species and novel gene identification. Average human gut microbiota is now better defined and has been estimated to exceed 1000 bacterial species [1, 2]. Bacteroidetes and Firmicutes represent predominantly in the

gut microbiota. Other phyla such as Proteobacteria, Actinobacteria, Cyanobacteria, and Verrucomicrobia, as well as methanogenic Archaea, mainly *Methanobrevibacter smithii*, are present in the gut microbiota only in a minority [3, 4]. The distribution of these phyla in the gut depends on a wide range of host factors including genetics, epigenetics, local immune response, oxygen gradient, dietary intake, and interactions among microbes.

Gastrointestinal microorganisms can influence host processes to impact host physiology, immunology, and metabolism. The composition, diversity, and functionality of the gut microbiota can alter signaling events between the microbiome and the host to influence gut homeostasis and host health [5]. Analysis of the microbiota can be performed in different states of diseases, and its results together with the application to animal experimental models can provide a simpler system in which the disease pathogenesis can be examined [5].

Reduction of the bacterial diversity and overall disbalance of the gut microbiota also known as dysbiosis is associated with many chronic diseases [6]. In some instances, gut microbiota alterations can affect intestinal permeability, allowing the transfer of lipopolysaccharide originating from the walls of gram-negative bacteria into the circulation, leading to endotoxemia and low-grade inflammation in different parenchymatous organs, resulting in metabolic disorders and chronic diseases [7].

The etiology of chronic diseases is often multifactorial, and gut microbiota is also one of the key factors. Current therapy for chronic diseases mostly does not reflect this fact, which limits its overall effectiveness. A better understanding of gut microbiota cross-talk mechanisms and their subsequent effects could provide new insights into the role of gut microbiota and dysbiosis in disease pathogenesis. This knowledge and technology can allow the development of potentially effective alternative approaches for preventive and therapeutic measures based on gut microbiota modulation [5, 8, 9]. More effective methods and biotherapeutics are needed for personalized and targeted gut microbiota modulation as supportive therapy for chronic diseases.

Targeted modulation of the gut microbiota represents an approach when specific bacterial strains have clinically proven effects of changes in the microbiota and human health. These bacteria are used in products to deliver specific predetermined effects for the host based on his disease and microbiota composition.
