| This paper focuses on selection of probiotics from bacterial strains which were isolated from intestinal tract of cultured healthy yellow catfish (Pseudobagrus fulvidraco), involving testing the antagonistic against commonly pathogenic bacteria of fish. For using probiotic in feed, characteristics and effects on growth performance, immune responses and intestinal microflora were studied.(1) Microflora in the tract of yellow catfish (Pseudobagrus fulvidraco) and in the water were studied in this paper. Results showed that:the logarithm of aerobic and facultive anaerobic bacteria of foregut, midgut and hindgut were 5.27±0.74,6.26±0.36,7.56±0.61. Obligate anaerobic bacteria numbers were 2 to 3 orders of magnitude more then them, and Bifidobacterium was the dominant species. The logarithm of bacteria in water was 4.80±0.73. Aeromonas, Enterobacteriaceae and Acinetobacter were dominant species in the tract of fish. They were also dominant species in water. In addition, the number of bacteria in the tract of Pseudobagrus fulvidraco suffered from hemorrhage disease had a a significant enhancement. Aeromonas increaced 18.3%-33.6%. And bacteria species decreased significantly.(2) 65 spore-producing strains were obtained from the tract of healthy yellow catfish using BHI culture medium. We get 53 strains of antagonistic bacteria against commonly pathogenic bacteria of fish (Cytophaga columnaris, Edwardsiella tard, Aeromonas hydrophila, Aeromonas sobria, Escherichia coli) by dot-inoculating method, and the diameter of inhibition zone of bacteria was up to 6.9-20.5 mm.3 strains were obtained from 8 strains which have better antagonistic effect by the method of Oxford-cup tests in the secondary screening. All of the 3 strains had wider activity against the 6 indicator bacteria. Physiological and biochemical (API 50CHB) identification showed that they belonged to Bacillus subtilis. Chosing F14 to make temperature and acid tolerance, antibiotics sensitivity, crude concentration activity tests and determin the viability in feed. It shows that, viability of F14 were 95.3% and 52.97% after cells be heated in 90℃and 100℃for 5 minutes. Up to 10 minutes,8.26% were spare after heating in 100℃. Tolerance ability of low pH value indicated that, in the pH range from 3.0 to 5.0, viability of F14 were more than 98% and 87% after be treated for 1 or 2 hours. F14 has highly sensitivity, medium sensitivity and tolerance to 8,4,3 kinds of antibiotics after making antibiotics sensitivity test to 15 kinds of antibiotics. Crude concentration of F14 have strong activity for straining bacteria. After 2 hours, the number of A.hydrophila declined rapidly, and we can not examine A.hydrophila after 24 hours. Sprayed F14 to the surface of feed and stored them at room temperature,4℃,-20℃, the livability were 75.26%, 87.42%,94.23% after 10 days and 57.73%,77.53%,87.63% after 20days.(3) A feeding experiment was conducted to evaluate the effects of probiotic on growth performance, immune responses and intestinal microflora. The bacterium was administered orally at 107 (A1),108 (A2),109(A3) cfu/g feed to yellow catfish (Pseudobagrus fulvidraco). The control group (A0) were fed feed without B. subtilis for the same period. Results showed that all experiments have enhanced the growth performance, however, fish fed A2 showed higher growth (P<0.05) rate compared with others. Diet with supplementation of probiotic could improve activity of phagocytic, lysozyme, Myeloperoxidase (MPO) in serum and increase content of serum complement (C3, C4). The probiotic could significantly decrease the number of Aeromonas and E.coli in intestine than those fed control diet. Bacillus was significantly increased (P<0.05) and fixed at a high level. Aerobic and facultive anaerobic bacteria was affected simultaneity. It demonstrated that Bacillus subtilis F14 could regulate intestinal flora, activate immun system, improve growth performance and resistance to disease.
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