Application of Bacillus subtilis in comprehensive prevention and control of streptococcus disease of tilapia

To provide research basis of using probiotic for comprehensively preventing and controlling streptococcus disease in tilapia aquaculture，effects of Bacillus subtilis as water addictive on disease resistance against Streptococcus agalactiae in tilapia were studied. B. subtilis was added to the water tanks of tilapia at final concentration of 1.0×104 CFU/ mL for group 1 and 1.0×105 CFU/mL for group 2，with no probiotic added to control tanks. Water quality parameters，fish immunity and antioxidant indicators were monitored. Fishes were challenged by S. agalactiae via intra-peritoneal injection at the end of experiment. The results showed that the concentrations of ammonia，nitrite nitrogen，total phosphorus in water of group 1 were lower than those of the control group，the indexes of group 2 were lower than those of the control group except the total phosphorus contents after 14 d and 21 d，and the ammonia content after 28 d were higher than those of the control group，and the nitrite nitrogen contents after 7 d，14 d and 28 d were higher than that of the control group. The activities of serum lysozyme（LZM），peroxidase（POD），alkaline phosphatase（AKP），total superoxide dismutase（T-SOD）and total antioxidant capacity（T-AOC）of tilapia increased in group 1 and group 2 compared to the control group in varying degrees. The cumulative mortalities of group 1 and group 2 were 42.22%，24.44%，and significantly lower than that of the control group（81.90%）. Appropriate concentration of B. subtilis as water addictive in tilapia could improve water quality and enhance immune of tilapia，thereby improve disease resistance against S. agalactiae in tilapia.