Abstract
Contribution of the exogenous enzymes supplemented by the gut-associated endosymbionts has been realized of late. Autochthonous gut microbiota is now considered as an important secondary factor that positively influences digestion and nutrient utilization of the host. Being one of the major colonizers within the gut of the fish, bacilli are metabolically active and produce a wide array of enzymes that play a vital role in the digestive processes of the host. The enzymes of nutritional importance produced by the gut bacteria may be categorized as (1) digestive enzymes, e.g., protease, amylase, and lipase, and (2) degradation enzymes, e.g., non-starch polysaccharide (NSP) - degrading enzymes, phytase, tannase, and chitinase. The probiotic attributes of the bacilli often lie within their capacity to produce this extensive range of enzymes that could act on diverse biological substrates of nutritional importance. Diverse species of the gut-associated bacilli with different enzymatic potential have been detected in varied fish species. Among these, Bacillus subtilis, Bacillus licheniformis, B. amyloliquefaciens, B. methylotropicus, Bacillus pumilus, and B. cereus may be mentioned. The present chapter will critically evaluate the occurrence of exo-enzyme-producing and gut-associated bacilli in fish and their contribution to fish nutrition.