Fibrolytic bacteria constitute a group of microorganisms that are able to process complex plant polysaccharides thanks to their capacity to synthesize cellulolytic and hemicellulolytic enzymes. Polysaccharides are present in plant cellular cell walls in a compact fiber form where they are mainly composed of cellulose and hemicellulose. Fibrolytic enzymes, which are classified as cellulases, can hydrolyze the β (1 ->4) bonds in plant polysaccharides. Cellulase and hemicellulase (also known as xylanase) are the two main representatives of these enzymes. Biological characteristics Fibrolytic bacteria use glycolysis and the pentose phosphate pathway as the main metabolic routes to catabolize carbohydrates in order to obtain energy and carbon backbones. They use ammonia as the major and practically exclusive source of nitrogen, and they require several B-vitamins for their development. They often depend on other microorganisms to obtain some of their nutrients. Although their growth rate is considered slow, it can be enhanced in the presence of considerable amounts of short-chain fatty acids (isobutyric and isovaleric). These compounds are normally generated as a product of the amino acid fermentative activity of other microorganisms. Because of their habitat conditions, most fibrolytic bacteria are anaerobic. Cellulolytic communities Most fibrolytic bacteria are classified as Bacteroidota or Bacillota and include several bacterial species with diverse morphological and physiological characteristics. They are normally commensal species which have a symbiotic relationship with different insect and mammal species, constituting one of the main components of their gastrointestinal flora. In fact, in herbivores each milliliter of ruminal content can reach about 50 million of bacteria of a great variety of genera and species. . Given the importance of industrial processing of plant fibers in different fields, the genomic analysis of fibrolytic communities in the gastrointestinal tract of different animals, may provide new biotechnological tools for the transformation of complex polysaccharides (including lignocellulytic biomass) . Applications So far, most applications are performed using enzymatic aqueous solutions containing one or more types of cellulases. Enzyme production for industrial use has its origins at the end of the nineteenth century in Denmark and Japan. An enzyme is a cellular product which can be obtained from animal and vegetable tissues, or through the biological activity of selected microorganisms. Enzymes are then used in different industrial processes. In order to produce enzymatic solutions for industrial applications, it is first necessary to obtain them in huge amounts and then, purify them to a certain extent; this makes the