| In recent years, frequent outbreaks of viral and bacterial diseases caused great economic losses to the shrimp farmers, which seriously hampered the sustainable development of the shrimp farming industry. The main measures in aquacultural diseases prevention and treatment is the use of antibiotics, which plays a positive role in disease treatment, at the same time, it also brings a series food and environmental safety issues. When people recognize the negative effects of antibiotics, people begin to look for natural, non-toxic, no residue, no pollution, safe and efficient disease prevention measures. Probiotics is a kind of micro-organisms, which can breed fast,decompose organic matter and other pollutants fast, has strong ability to adapt to the environment. At present, probiotics has been widely used as a safe and effective disease prevention measures in aquaculture.12 Bacillus strains isolated from shimp aquaculture environment has the ability to improve water quality. By determinating the extracellular enzyme activities and evaluating the potential pathogenicity. Results showed that 12 strains had the ability to secrete extracellular enzymes. HNLS-1, HNLD-1, HNLD-2 and HNSY-1 were ?-hemolysis, the other 8 strains were significant ?-hemolysis. Result indicate that,HNLS-1, HNLD-1, HNLD-2 and HNSY-1 could used as potential probiotic strains alternatives in Litopenaeus vannamei.The effects of HNLS-1, HNLD-1, HNLD-2 and HNSY-1 in vitro were confirmed by a series tests, such as tolerance to bile salts, resistance to artificial gastric juice and artificial intestinal fluid, antibacterial activity, surface hydrophobicity and auto-aggregation. In vitro assessment showed that strains HNLS-1, HNLD-1,HNLD-2 tolerated 0.6% of bile salt, however, HNSY-1 was very sensitive to bile salt concentration. The viable counts of strain HNLS-1?HNLD-1 and HNSY-1 did not change after 180min of growing in simulated gastric fluid and HNLD-2 decreased 1 log value after 60min. The viable counts of 4 strains did not decrease in simulated small intestinal fluid after 180min.Bacillus sp.Strains HNLS-1?HNLD-1 and HNSY-1 which inhibited the growth of pathogenic Vibrio, HNLD-1 and HNSY-1 respectively inhibited growth of V. parahaemolyticus and V. vulnificus with the maximum inhibition halos of 14.53 and 11.43 mm diameter. HNLD-2 showed no inhibition to these Vibrio strains. The cell surface hydrophobicity of HNLS-1 and HNLD-1 was higher than HNLD-2 and HNSY-1, reached 64% and 73.2%respectively. With an increase in incubation time the auto aggregation increases rapidly, HNLS-1 and HNLD-1 showed a significant higher auto aggregation degree after 5 h incubation compared with HNLD-2 and HNSY-1, reached 58.5% and 52%respectively. In summary, Bacillus sp. strains HNLS-1 and HNLD-1 have better in vitro probiotic properties, it is meaningful to further research its effect in aquatic animal nutrition and diet application as feed additive.HNLS-1 and HNLD-1 were identified as Bacillus licheniformis and Bacillus flexus by Gram stain, bacterial physiological and biochemical characteristics and 16S rDNA sequence analysis.This research studied the effects of HNLS-1 and HNLD-1 on growth performance, immunity, disease resistance and water improvement of Litopenaeus vannamei postlarvae. The shrimps were divided randomly into four groups and each group was fed one of four diets for 20 days: a basal diet without probiotics (control),or the basal diet supplemented with HNLS-1 , HNLD-1, or a mixture of HNLS-1 and HNLD-1. After 20 days, shrimp receiving the diets supplemented with probiotics showed significantly higher survival rate and specific growth rate, and lower concentration of ammonia nitrogen, COD and Vibrio spp. counts than the control(P<0.05). The stresses resistance in fresh water, 40ppt salt water and 300mg/L nitrite were better than the control. The survival rate after Vibrio harveyi infection of live treatment groups were significantly higher than the control (P<0.05) after 1 weeks. The protease, lipase, alkaline phosphatase and lysozyme activity of each treatment groups was significantly higher compared with the Control (P < 0.05). The peroxidase activity of HNLS-1 and HNLD-1 was also significantly increased (P <0.05). ). In conclusion,both HNLS-1 and HNLD-1 show probiotic effects, therefore they can be used as probiotics in the farming of Litopenaeus vannamei.In conclusion, HNLS-1 and HNLD-1 show positive effect on the growth, disease resistance and water improvement of Litopenaeus vannamei, therefore, it settles the theoretical and applied basement of development and manufacture of probiotics for Litopenaeus vannamei culture. |
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