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Summary

Tannins are the most abundant among the plant-derived antinutrients that bind readily with protein and other macromolecules to form indigestible complexes, thereby reducing the nutritional value of the plant feedstuffs. Presence of tannase-producing gut microbiota in herbivorous animals has been suggested to overcome the antinutritional effects of tannins. However, this topic has been less investigated in herbivorous/omnivorous fish species. The present study was undertaken to evaluate the presence of tannase-producing autochthonous microbiota in the gastrointestinal (GI) tracts of some culturable freshwater teleosts and to identify most promising tannase-producing strains by molecular methods. Isolation and enumeration of tannase-producing autochthonous microbiota have been carried out in the gut of ten culturable freshwater teleosts, namely catla (Catla catla), silver carp (Hypophthalmichthys molitrix), rohu (Labeo rohita), grass carp (Ctenopharyngodon idella), mrigal (Cirrhinus mrigala), common carp (Cyprinus carpio), bata (Labeo bata), kalbasu (Labeo calbasu), tilapia (Oreochromis mossambicus), and Nile tilapia (Oreochromis niloticus). Culturable heterotrophic and tannase-producing microbial populations evaluated on tryptone soya agar and selective tannic acid agar media, respectively, revealed the maximum in the hindguts of all fish species studied. Out of 72 tannase-producing colonies, 18 randomly selected isolates were maintained as pure cultures and evaluated quantitatively for tannase production. Among these, four most promising tannase producers were identified by 16S/26S rDNA sequencing following nucleotide blast and deposited in the National Centre for Biotechnology Information (NCBI) GenBank. The strain LR01 isolated from rohu was a bacterium, Enterobacter asburae (GenBank Accession No. GU939631). However, the strains CM02, OM01 and LR03 isolated from mrigal, tilapia and rohu were yeasts and identified as Pichia kudriavzevii (GenBank Accession No. GU939629), Candida tropicalis (GenBank Accession No. GU911469) and Candida parapsilosis (GenBank Accession No. GU939630), respectively. To the authors' knowledge, the present study is the first to report tannase-producing autochthonous microbiota in the gut of freshwater teleosts. Tannin-degrading microbiota detected in the present study may endow the fish with some ecological advantages by enabling them to overcome the anti-nutritional effects of plant tannins.