Effects of the Probiotic Psychrobacter sp. B6 on the Growth, Digestive Enzymes, Antioxidant Capacity, Immunity, and Resistance of Exopalaemon carinicauda to Aeromonas hydrophila.

Probiotics, known to improve the water quality and the host's intestinal microbial balance, has gained more and more attention in recent years. The effects of Psychrobacter sp. B6 on the growth and immunity of Exopalaemon carinicauda were investigated in this study. Psychrobacter sp. B6 was sprayed to the basal diet with four different levels (0 [basal diet], 5 × 105, 5 × 107, and 5 × 109 CFU/100 g diet) and were fed to E. carinicauda (average weight 1.15 ± 0.04 g) for 30 days. At the end of the feeding trial, shrimps were immersed in seawater contaminated with 106 CFU/mL pathogenic Aeromonas hydrophila for 2 h and then the cumulative mortality was calculated after 14 days observation. The results showed that the weight gain rate, survival rate, and specific growth rate of E. carinicauda were significantly increased with the increasing dietary level of Psychrobacter sp. B6. The activities of digestive enzymes (α-amylase and chymotrypsin) were significantly increased (P < 0.05) in the groups fed with Psychrobacter sp. B6, and the highest activities of digestive enzymes were detected in the 5 × 109 CFU/100 g diet group. The activities of antioxidant enzymes (catalase, peroxidase, and superoxide dismutase) in probiotics treated shrimp were significantly higher than those in the control shrimp, with the highest activity in 5 × 109 and 5 × 107 CFU/100 g diet group separately. At the same time, the activities of immune-related enzymes (alkaline phosphatase and lysozyme) were significantly affected by the dietary B6 content, and the highest activity of immune-related enzymes was found in shrimps fed with 5 × 107 CFU/100 g diet. The relative expression levels of CTL (C-type lectin), MBL (mannose-binding lectin), SPI (serine protease inhibitor), and ProPo (prophenoloxidase) in hepatopancreas of E. carinicauda with 5 × 109 CFU/100 g diet were significantly higher than those in the control. Moreover, cumulative mortality (22.22%) post-challenge with A. hydrophila was the lowest in 5 × 109 CFU/100 g diet. The results suggested that Psychrobacter sp. B6 could effectively promote the growth, immunity, antioxidant capacity, and disease resistance of E. carinicauda. This study provided a reference for the study on the artificial breeding of E. carinicauda.