During coevolution, the microbial community has become an integral and vital part of the alimentary canal in many invertebrates and vertebrates, including fish [32]. Enteral microbiota plays an important role in the regulation of general metabolism, providing protective functions and digestion in the host body [24, 25, 29]. To date, the key factors influencing the formation of the fish enteral microbiota, such as food spectrum, age, season, water temperature, etc., have been identified [30, 31]. According to some authors, food type is one of the most important factors influencing the structure of the fish enteral microbiota [30-32]. During the microbiocenosis formation in the alimentary canal in the host organism, two groups of microorganisms can be distinguished: the temporary allochthonous microbiota and the autochthonous microbiota that constantly inhabits its mucous membrane surface. As a rule, the allochthonous and autochthonous microbiota enters the alimentary canal from the environment with water and food [31]. At different stages of ontogenesis, the enteral microbiota formation is well studied in fish bred in pond farms where the external environment parameters are monitored [22, 28]. Some researches also address the ways of microbiota formation in the fish alimentary canal [30].
There are various approaches to studying enteral microbiota, such as molecular genetic methods, and techniques for culturing bacteria with physiological and biochemical markers used to identify them. Culturing bacteria on selective media is a traditional research method. However, it has been shown that more than 70% of the enteral microbiota can't be cultured and identified by this method. [13]. To solve the problem of identifying nonculturable taxa, molecular genetic methods are currently widely used. Such approaches are based on the analysis of 16S ribosomal RNA (rRNA) genes and allow identifying nonculturable bacteria. Variable 16S rDNA regions are unique for many bacterial species and strains and are widely used for their identification [14].