The gastrointestinal (GI) tract of an animal consists of a very complex and dynamic microbial ecosystem that is very important from a nutritional, physiological and pathological point of view. A wide range of microbes derived from the surrounding aquatic environment, soil/sediment and feed are found to colonize in the GI tract of fish. Among the microbial groups, bacteria (aerobic, facultative anaerobic and obligate aneraobic forms) are the principal colonizers in the GI tract of fish, and in some fish, yeasts are also reported. The common bacterial colonizers in the GI tract of freshwater and marine fish include Vibrio, Aeromonas, Flavobacterium, Plesiomonas, Pseudomonas, Enterobacteriaceae, Micrococcus, Acinetobacter, Clostridium, Fusarium and Bacteroides, which may vary from species to species as well as environmental conditions. Besides, several unknown bacteria belonging to Mycoplasma, Arthrobacter, Brochothrix, Jeotgailbacillus, Ochrobactrum, Psychrobacter and Sejongia species in the GI tract of different fish species have now been identified successfully using culture‐independent techniques. Gnotobiotic and conventional studies indicate the involvement of GI microbiota in fish nutrition, epithelial development, immunity as well as disease outbreak. This review also highlights the need for manipulating the gut microbiota with useful beneficial microbes through probiotic, prebiotic and synbiotic concepts for better fish health management.

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This investigation was undertaken to characterize the abundance and genera-wise diversity of cyanobacteria in the rice rhizosphere and nitrogen-fi xing ability of the isolated strains. The cyanobacterial strains belonging to the genera Nostoc and Anabaena comprised 80% of the rhizosphere isolates, which were also effi cient in enhancing the germination and growth of wheat seeds and exhibited signifi cantly high protein accumulation and IAA production. Distinct profi les for the cyanobacterial strains were obtained on amplifi cation with extended Hip 1 primer -HipTG, indicative of the diversity among these strains. Our investigation helped in identifying promising cyanobacterial isolates from the rhizosphere of rice, which can be utilized in developing effi cient plant growth promoting cyanobacterial inoculants.

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