Characterisation of Extracellular Enzyme-Producing Microorganisms From the Gut of Nile Tilapia, Oreochromis niloticus (Linnaeus, 1758)

The current priority in the aquaculture industry is to replace fish meal with cheaper but efficient protein sources like plant-based feedstuffs to support global expansion and sustainability. Hence, research has focused on finding novel ways to increase the digestibility of plant-based diets. Exogenous enzymes have emerged as an excellent alternative to improve nutrient digestibility of the diet in animals, including fish. The present study aims to explore exogenous enzyme-producing bacteria in the gastrointestinal (GI) tract of Nile tilapia (Oreochromis niloticus (Linnaeus, 1758)). Two Gram-negative enzyme-producing (amylase, xylanase, and lipase) bacterial strains viz. Aeromonas veronii ONKP1 (GenBank accession no. MN602971) and Stenotrophomonas maltophilia ONKP2 (GenBank accession no. MN602972) were identified by biochemical tests and 16s RNA analysis. They are capable of utilising citrate, maltose, glucose, and gelatine. Besides, A. veronii ONKP1 can utilise mannitol, sucrose, and lactose, whereas S. maltophilia ONKP2 is catalase and urease positive. Aeromonas veronii ONKP1 was superior in terms of enzyme production to S. maltophilia ONKP2. Stenotrophomonas maltophilia ONKP2 is a rarely reported strain, specifically from healthy fish. Amylase, xylanase, and lipase could be used as feed additives for fermenting plant products and producing prebiotics like xylooligosaccharides. Further, this study might help understand the role of gut-associated bacteria viz. A. veronii and S. maltophilia, in fish nutrition.