Effects of the probiotic, Bacillus subtilis E20, on the survival, development, stress tolerance, and immune status of white shrimp, Litopenaeus vannamei larvae

In this study, the probiotic, Bacillus subtilis E20, isolated from the human health food, natto, was used for white shrimp, Litopenaeus vannamei, larvae breeding to improve the larval survival rate and development by adding probiotic to the rearing water at (control), 108, and 109 cfu L−1 salt water once every 3 days during the 14 days of breeding experiment. Thereafter, stress tolerance and immune status of postlarvae were evaluated. Shrimp larval development was significantly accelerated after adding the probiotic to the larval rearing water at a level of 109 cfu L−1. The survival rate of larvae was significantly higher in the treatment with 109 cfu L−1 compared to the control and the treatment with 108 cfu L−1 after all larvae had metamorphosed to postlarvae. Adding the probiotic to the shrimp larvae rearing water produced a weak inhibition of bacterial growth by an analysis of the total bacterial count and presumptive Vibrio count. For stress tests, no postlarvae died when they were reared in water in which the temperature was decreased from 30 to 2 °C at a rate of 0.1 °C min−1. Postlarvae had significantly lower cumulate mortality in the treatments with 108 and 109 cfu L−1 compared to the control when they were suddenly exposed to fresh water and 60‰ salt water. A significant decrease in the cumulative mortality of postlarvae treated with the probiotic at a level of 109 cfu L−1 was recorded after the sudden transfer to 300 mg L−1 nitrite-N compared to the control and treatment with 108 cfu L−1. The analysis of immune-related gene expressions showed that the gene expression of prophenoloxidase I, prophenoloxidase II, and lysozyme of larvae were significantly increased after being reared in probiotic-containing water at the levels of 108 and 109 cfu L−1. However, no significant difference in serine proteinase or glutathione peroxidase gene expressions was recorded in this study. It is therefore suggested that 109 cfu L−1 of probiotic, B. subtilis E20 adding to rearing water for shrimp larva breeding.