Labeo bata

Abstract: Background. The information on gut microflora in fish is scanty and there is a paucity of knowledge regarding microbial enzyme activity in fish gastrointestinal tracts. Although some information is available on the enzymeproducing bacteria in fish digestive tracts, almost nothing is known about their distribution in different regions of the gut. In the present study, an attempt has been made to investigate the distribution of enzyme-producing microflora in the foregut and hindgut regions of seven culturable freshwater teleosts. Materials and Methods. Isolation and enumeration of aerobic bacterial flora in the foregut and hindgut regions of the gastrointestinal tracts of seven freshwater teleosts of different feeding habits, namely rohu, Labeo rohita; catla, Catla catla; mrigal, Cirrhinus mrigala; bata, Labeo bata; orange-fin labeo, Labeo calbasu; Nile tilapia, Oreochromis niloticus; and climbing perch, Anabas testudineus, have been carried out. Microbial culture of the gut mucosa on selected nutrient media, following the enrichment culture technique, was done for bacterial isolation. Bacterial isolates were qualitatively screened on the basis of their extracellular enzyme-producing ability. The selected strains were further quantitatively assayed for amylase, cellulase and protease activities. Results. In general, bacterial population was lower in the foregut region of all the seven species of fish examined. Amylolytic strains were present in higher densities in the foregut region of orange-fin labeo and bata (12.20 × 103 CFU · g–1 gut tissue and 11.50 × 103 CFU · g–1 gut tissue, respectively) in comparison to the hindgut region. The cellulolytic population exhibited maximum densities in the hindgut region of bata (7.20 × 103 CFU · g–1 gut tissue) followed by the foregut region of the same fish (5.50 × 103 CFU · g–1 gut tissue). Amylolytic and cellulolytic bacterial flora was not detected in both the fore and hindgut regions of climbing perch. Proteolytic bacterial flora was found in all the species of fish studied and the maximum count was observed in the hindgut region of bata (13.40 × 103 CFU · g–1 gut tissue), orange-fin labeo (9.00 × 103 CFU · g–1 gut tissue), Nile tilapia (8.30 × 103 CFU · g–1 gut tissue) and climbing perch (7.20 × 103 CFU · g–1 gut tissue). Minimum count of proteolytic bacterial flora was observed in the foregut region of all the fishes studied. Peak amylase and cellulase activities were exhibited by bacterial strains isolated from the foregut of orange-fin labeo (266.43 ± 0.15 U) and the hindgut of bata (64.01 ± 0.42 U), respectively. Maximum protease activity was exhibited by a strain isolated from the hindgut region of orange-fin labeo (44.33 ± 0.09 U), followed by the strains isolated from the hindgut regions of climbing perch (32.87 ± 0.12 U), bata (29.71 ± 0.11 U), and Nile tilapia (29.46 ± 0.11 U). Conclusions. The results of the present study indicate that there is a distinct microbial source of digestive enzymes apart from the endogenous sources in fish digestive tracts. The enzyme-producing bacteria isolated from the digestive tracts can be beneficially used as a probiotic while formulating aquafeeds, especially in the larval stages. However, further investigations are required to determine if the addition of such isolates to fish feeds do, in fact, provide some kind of benefit to the fish involved before advocating their use.

Abstract: 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.