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The hepatopancreatic disease in shrimp has caused great economic losses in recent years, and it is necessary to identify the responsible pathogen and establish control methods. The present study aimed to identify the primary pathogenic bacteria in diseased Litopenaeus vannamei hepatopancreas (HP) and to explore the protective effects of functional bacteria on shrimp. Four pathogenic Vibrio spp. were isolated and identified from shrimp with hepatopancreatic disease, of which Vibrio harveyi HAH could be the primary pathogen causing higher rates of mortality (~ 73%) with observed symptoms. In addition, HAH had five virulence-related genes: luxR, flaB, toxR, vhhA, and vhhB. Anti-Vibrio bacterium M146 and quorum quenching (QQ) bacterium W1B were screened and used as a feed supplement. The results showed that M146 and W1B (dosed at 1.0 × 107 CFU/g) decreased the mortality of shrimp after challenge with HAH. Supplementation with M146 significantly increased the activities of alkaline phosphatase (AKP) and lysozyme (LZM), whereas supplementation with W1B had no significant effect. AKP and acid phosphatase (ACP) activities of shrimp supplemented with W1B showed a significant increase after infection with HAH, whereas a decrease in the activities of AKP and superoxide dismutase (SOD) was observed in the negative control (NC). Supplementation with M146 and W1B significantly decreased the relative messenger RNA expression of Relish and TRAF6. However, after infection with HAH, the M146 and W1B groups exhibited a different pattern of gene expression regulation compared with that in the NC group, which might suggest weakening of the virulence of the pathogen. The results suggest that these two bacteria can protect L. vannamei and that M146 not only inhibited the growth of HAH but also had an immunostimulatory effect on shrimp. On contrast, W1B did not improve the immunity of shrimp, so it is speculated that it may protect the host by affecting the quorum sensing (QS) system of pathogenic bacteria.

This study investigates the acute responses of pacific white shrimp Litopenaeus vannamei to an abrupt reduction of salinity in osmosis-, metabolism-, and immune-related enzyme activities in the hemolymph of L. vannamei previously reared at 56‰ salinity. Three salinity reductions (56 to 54‰, 56 to 52‰, and 56 to 48‰) were used, and L. vannamei was directly transferred from the salinity of 56 to 54‰, 52‰, and 48‰, respectively. The hemolymph was collected from five shrimp in each group at 1 h, 6 h, 12 h, 18 h, 24 h, 36 h, and 48 h to detect the change of ATPase (including Na+/K+-ATPase and T-ATPase), alkaline phosphatase (AKP), acidic phosphatase (ACP), and superoxide dismutase (SOD) activities. Our results showed that no shrimp death occurred after an abrupt reduction of salinity. However, ATPase, SOD, ACP, and AKP enzyme activities in three salinity reductions all changed significantly, and the activities of T-ATPase and Na+/K+-ATPase showed a single peak trend during the process of experiment. The SOD activity peaked at 24 h after the salinity was decreased. ACP activities remained an increasing trend within 48 h after salinity was decreased in the three salinity treatments. A single peak trend was found in AKP activity and peaked at 18 h and then decreased significantly. The results indicate that an abrupt decrease in salinity significantly affected ATPase, SOD, ACP, and AKP activities. Osmotic, metabolism, and immune-related enzyme activities of L. vannamei are sensitive parameters to respond to an abrupt drop of salinity.