Bacterial symbiosis in the fish gut and its role in health and metabolism

The bacterial community in the fish gut is very dense compared to surrounding water, which suggests that gastrointestinal tract (GIT) provides a favorable ecological niche for survival. GIT bacteria can be broadly divided into two groups; autochthonous (able to colonize on the mucosal surface) and allochthonous (free living). From the host's point of view, autochthonous bacteria are considered to be more important than allochthonous, as they provide both nutritional as well as disease prevention support to the host. Among their several functions, the autochthonous bacteria are believed to produce several types of extracellular enzymes, block the attachment site for pathogens and secrete a wide range of bacteriocins. Most of the bacterial species in the gut are non-culturable and thus several types of sophisticated techniques such as Denaturing Gradient Gel Electrophoresis (DGGE), Temperature Gradient Gel Electrophoresis (TGGE) and Next Generation Sequencing (NGS) have been introduced to explore the microbial communities in gut. In this present review, we have summarized the impact of gut bacteria in fish with special emphasis on extracellular enzyme production by gut microbiota, bacterial composition, mechanism of attachment in epithelial surface and their role in disease prevention.