Abstract:The present communication deal with the identification of probiotic strain isolated from the intestine of Cirrhinus mrigala (Mrigal) on the basis of phylogenetic analysis and nucleotide homology and was found similar to Bacillus cereus strain SL1. The strain was further investigated for its probiotic abilities; acid and bile tolerance, autoaggregation, coaggregation and hydrophobicity. A dietary experiment of 90 days was carried out to examine its effect on the growth, digestive enzyme activity and survival of C. mrigala. Four isonitrogenous and isocaloric diets were prepared and B. cereus was incorporated in the experimental diets; T1, T2 and T3 at three different levels in the proportion of 2 × 10 4 , 2 × 10 5 and 2 × 10 6 cells 100 g -1 of feed respectively, along with a control diet TC without probiotic. After 90 days, the group of mrigal fed on probiotic supplemented diets showed significantly (P<0.05) better growth, specific growth rate (SGR), feed conversion ratio (FCR) than those fed with basal diet (control) with best result for diet T2 (P<0.05). Values of digestive enzyme activities (protease, amylase and cellulose) and carcass protein were also significantly (P<0.05) higher and excretion of metabolites (ammonia and phosphate) were lower in groups fed with diet T2. When probiotic supplemented diet fed fish were challenged with pathogenic Aeromonas hydrophila by immersion for 10 days, all probiotic fed groups showed high survival in comparison to group fed on control diet without probiotic supplementation indicating immunostimulating effect of probiotic in diets of C. mrigala.

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 Cirrhinusmrigala. 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%. Amongst the isolates gram positive were maintained in nutrient agar slants at 4°C. Of these, eight strains were replica-plated on agar seeded with Aeromonashydrophila and only one strain CM2 (Cirrhinus 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 (272.4±1.5), high apparent protein digestibility ((79.9±0.30)%) and low food conversion ratioin 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 probiotic indicating the extracellular enzyme production by B. cereus. These results indicate that probiotic 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.