Background. Although, presence of tannase-producing and/or tannin tolerant gut bacteria has been documented in ruminants and non-ruminant herbivores, the topic is rarely addressed in fish. The present study aimed at enumeration of autochthonous tannase-producing bacteria in the gut of freshwater teleosts. Materials and methods. This study covered seven freshwater teleost fish species: rohu, Labeo rohita (Hamilton, 1822); catla, Catla catla (Hamilton, 1822); mrigal, Cirrhinus mrigala (Hamilton, 1822); grass carp, Ctenopharyngodon idella (Valenciennes, 1844); common carp, Cyprinus carpio Linnaeus, 1758; silver carp, Hypophthalmichthys molitrix (Valenciennes, 1844); and Nile tilapia, Oreochromis niloticus (Linnaeus, 1758). Gastrointestinal (GI) tracts of the fish studied were divided into proximal (PI) and distal (DI) parts, homogenized and plated onto Tryptone Soya Agar (TSA) plates. The pure colonies were spotted on selective tannic acid (TA) agar plates to determine the tannase-producing bacteria. Extracellular tannase-producing capacity of the isolates was determined through qualitative and quantitative assay using TA media plates or broth, respectively at three different pH levels (5.5, 7.0, and 8.5). Further, 16S rRNA gene fragments of the promising tannase-producing bacteria were sequenced, aligned, analysed, identified, and deposited to the GenBank. Results. Totally 685 strains were isolated on TSA plates, of which 116 strains (37 from PI and 79 from DI) grow on TA media and were defined as tannase-producers. The highest percentage of tannase-producing strains was noticed in the DI regions of grass carp, Ctenopharyngodon idella (38.98%), and tilapia, Oreochromis niloticus (37.74%). The lowest frequency of tannase-producing bacteria was revealed in PI region of catla, Catla catla (2.12%). The intestine of Indian major carps was relatively poorly colonized by tannase-producing bacteria compared to that of exotic carps. Evaluation of tannase-producing capacity revealed that the majority of the isolates exhibited maximum extracellular tannase production at pH 7.0. Quantitative evaluation, showed highest tannase activity by strain HMT1 (0.28 ± 0.001 U) isolated from silver carp, Hypophthalmichthys molitrix, followed by strains ONH2Ph (0.19 ± 0.005) and ONH13B (0.17 ± 0.009 U) isolated from tilapia. Analyses of the 16S rRNA partial gene sequences revealed that strains ONH2Ph and ONH13B showed high similarity to Bacillus subtilis (KP765736) and Brevibacillus agri (KP765734), respectively. Whereas, strain HMT1 was most closely related to Klebsiella variicola (KP765735). Conclusion. The study revealed existence of tannase-producing bacterial symbionts within fish GI tracts. Tannin-degrading bacteria detected in the presently reported study might aid in overcoming the anti-nutritional effects of dietary tannins within fish gut.

freshwater teleosts, gut bacteria, autochthonous bacteria, tannase