Research On Screening, Characterization And Ecological Effects Of Antagonistic Bacteria Against Streptococcus Iniae

streptococcosis is a s ignificant pathogen of talipia that has caused serious tilapia and economic losses. At present chemotherapeutics was the main means to prevent and cure diseases of Streptococcus iniae, but chemotherapeutic could cause much problem. The worsened aquaculture environment is one of the most important factors that cause increasing the mortality of tilapia. Increasing with the demand for environment friendly aquaculture, disregarding previous traditional means of disease control and finding a way that could improve the aquaculture wastewater and inhibit the growth of pathogenic bacteria is very important. The paper screen antagonistic bacteria against Streptococcus iniae from bacteria that had the ability of purification of stimulant aquaculture wastewater and were isolated from water. The antifungal activity, pHysical and chemical character of antagonistic substance, and fermentation conditions were investigated. At the same time, ecological control effect of antagonistic bacteria N0906against Streptococcus iniae in tilapia mossambica cultivation was tested. This would help develop strategies for biocontrol of streptococcosis in tilapia.7strains of antagonistic bacteria against Streptococcus iniae were isolated from our laboratory preserved bacteria which had the ability of purification of stimulant aquaculture wastewater and were isolated from water. The most compatible strain was chosen for growth curves, co-agglutination assay and ability of cleanse effects on the aquaculture wastewater. The result showed:7isolated strains had more advantage in RI than Streptococcus iniae, shorter doubling time were found at L09017, C0909and N0906indicating the growth advantage in the intestine of Streptococcus iniae; Only3(L0914, C0909, N0906)of7strains has co-agglutination effect with Streptococcus iniae and the co-agglutination ratio was3.03%,4.11%,1.85%respectively. All the7strains have the ability of cleanse effects on the aquaculture wastewater, but C0909, L0914has less advantage in degrading ammonia-nitrogen. In conclusion, N0906was more moderate to be as biocontrol bacteria against streptococcosis in tilapia and have good development and research value. Nocardia0906was chosen for further research and the ability of inhibition was tested. It showed that there was no antibacterial effect during the initial stage of cellular growth. The antagonistic activity of Nocardia0906began to increase in exponential pHase, reached maximum in the stationary and death pHases. The inhibition valence of the fermented both reached8AU/ml. The antibacterial substances were well in thermal stability, ultraviolet stability and acidity/alkalic stability. But they were non-tenderness about protease K and trypsin. The antibacterial substances were well soluble in water solvents but opposite in lipid solvents. The sediment had antagonistic activity after the treatment with the final concern of40%(NH4)2SO4The research intimated that antagonistic substance was a kind of protein.In order to improve the activity of Nocardia0906to producing antagonistic substance, the most suitable fermentation conditions of the strain were studied through single factor and orthogonal experiment. The result showed that medium with2.5%of solublestarch,0.5%of (NH4)2SO4,0.085%of K2HPO4and20%of potato extract, and control the temperature at32℃, pH value at8.5, shaker at150rpm for96hours, the optimal seed aged12hours, the inoculation volume of2%, the quantity of medium50ml in a100ml flask, the inhibition valence of the fermented both reached to the highest. Under such conditions, the concentration of the antagonistic substance which was produced from Nocardia0906improved25.6%.The research of protecting tilapia from streptococcicosis showed that the group treated with106CFU/ml Nocardia0906had lower concentration of NH3-N, NO2--N than the control group (P<0.05) and decreased by85.51%,54.38%respectively. The activity of LSZ, SOD, AKP and complement C3in the tissue were significantly higher than those of the control group (P<0.05) and increased by23.39%,39.19%,101.13%,65.16%respectively. The attenuation rate of bacterium in culture water were lower than the control group (P<0.05) and the final mortality of tilapia were much lower than the control group (54.17%, p<0.05).The pHotosynthetic bacteria at suitable concentration can have the better effect of though purifying the water of culture water, enhancing body immunity and inhibiting the growth of and have better further development and application.