| Summary: | The present studies were conducted to isolate, select, identify and characterize gut bacteria as antimicrobial and growth promoting agent for the feed of economically important fish Cirrhinus mrigala. Intestinal microflora were isolated, counted, and identified, and their in vitro antibacterial properties were determined. The results have revealed that occurrence of Gram-negative rods was around 75% and of Gram-positive rods was 25%. Among the isolates Gram-positive were maintained in nutrient agar slants at 4°C. Of these, eight strains were replica-plated on agar seeded with Aeromonas hydrophila and only one strain CM2 (C. mrigala 2) exhibited antibacterial properties in vitro showing inhibition against fish pathogen by well diffusion assay. This isolated strain was identified as Bacillus cereus. This bacterium was mass cultured and incorporated in the pelleted diet (40% protein and 18 kJ g−1 gross energy) of C. mrigala to investigate its effect on growth performance, digestibility, nutrient retention and activities of digestive enzymes. The results of feeding trial revealed significantly (P<0.05) high growth performance in terms of specific growth rate, growth percent gain in body weight (BD) (272.4±1.5)%, high apparent protein digestibility ((79.9±0.30)%) and low food conversion ratio in the group of fishes fed on diet containing B. cereus in comparison to the fishes fed on diet without probiotics. The carcass composition also revealed high accumulation of proteins ((15.28±0.15) g 100 g−1) in fishes fed on diet containing probiotics. Intestinal enzyme activities of protease, amylase and cellulase were also significantly (P<0.05) high in the group of fishes fed on diet supplemented with probiotics indicating the extracellular enzyme production by B. cereus. These results indicate that probiotics stimulate the digestion through the supply of digestive enzyme and certain essential nutrients to animals. Also significantly (P<0.05) low excretion of metabolites, i.e., ammonia excretion and phosphate production (mg kg−1 BW d−1) was observed in the fishes fed with probiotic based diets. Thus, B. cereus appears to be a promising candidate for ptobiotic applications which can enhance growth performance and nutrient retention in C. mrigala.
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