Responses of Mytilus galloprovincialis hemocytes to environmental strains of Vibrio parahaemolyticus, Vibrio alginolyticus, Vibrio vulnificus

Marine bivalves are exposed to different types of bacteria in the surrounding waters, in particular of the Vibrio genus. In the hemocytes of the mussel Mytilus spp. immune responses to different vibrios have been largely characterized. However, little information is available on the hemocyte responses to human pathogenic vibrios commonly detected in coastal waters and bivalve tissues that are involved in seafood-borne diseases. In this work, functional parameters of the hemocytes from the Mediterranean mussel M. galloprovincialis were evaluated in response to in vitro challenge with different vibrios isolated from environmental samples of the Adriatic sea (Italy): V. parahaemolyticus Conero, V. alginolyticus 1513 and V. vulnificus 509. V. parahaemolyticus ATCC 43996 was used for comparison. At the 50:1 bacteria hemocyte ratio, only V. parahaemolyticus strains induced significant lysosomal membrane destabilisation. Stimulation of extracellular lysozyme release, total ROS, O₂^- and NO production were observed, although to different extents and with distinct time courses for different vibrios, V. vulnificus 509 in particular. Further comparisons between V. parahaemolyticus Conero and V. vulnificus 509 showed that only the latter induced dysregulation of the phosphorylation state of p38 MAP Kinase and apoptotic processes. The results indicate that mussel hemocytes can mount an efficient immune response towards V. parahaemolyticus and V. alginolyticus strains, whereas V. vulnificus 509 may affect the hemocyte function. This is the first report on immune responses of mussels to local environmental isolates of human pathogenic vibrios. These data reinforce the hypothesis that Mytilus hemocytes show specific responses to different vibrio species and strains.

Dietary administration of saponin stimulates growth of the swimming crab Portunus trituberculatus and enhances its resistance against Vibrio alginolyticus infection

The immunostimulatory role of dietary saponins on swimming crabs was investigated under field conditions. Portunus trituberculatus were fed diets enriched with Quillaja saponin (QS) at 150, 300 and 450 mg kg^-1. They had an enhanced growth rate and increased resistance against Vibrio alginolyticus compared to crabs not fed with QS. Significant effects were observed on the average body weight, percentage weight gain and specific growth rate (p < 0.05). Total hemocyte and hyaline cell counts of P. trituberculatus fed with 300 and 450 mg kg^-1 saponin in their diets significantly increased (p < 0.05) compared to the control. Phenoloxidase, superoxide dismutase, catalase and glutathione peroxidase activities significantly increased in response to the incorporation of QS in the diet. However, the respiratory burst activity did not increase significantly. The phagocytic activity was significantly enhanced at 300 and 450 mg kg^-1 of saponin. QS diets can enhance growth of P. trituberculatus and its immune resistance against V. alginolyticus. Dietary administration of saponin may help to control diseases and improve production in the crab aquaculture industry.

DNA shuffling approach for recombinant polyvalent OmpAs against V. alginolyticus and E. tarda infections

Molecular breeding via DNA shuffling directs the evolution of vaccines with desired traits. In the present study, polyvalent OmpA vaccines were generated by DNA shuffling of five ompA genes from four species of bacteria Vibrio parahaemolyticus, V. alginolyticus, Edwardsiella tarda and Escherichia coli. First, a new hybrid OmpA was constructed using VA0764 primers and used for construction of a prokaryotic expressing library PompAs-FV containing 84 ompAs, which were validated by PCR and SDS/PAGE. Then, the 84 ompAs were used to construct a eukaryotic expressing library EompAs-FV for preparing DNA vaccines. Third, extracellular bacterium V. alginolyticus challenge post active immunization using these DNA vaccines was carried out to identify genes with high immunoprotection. Among the 84 ompAs, 17 showed higher or equal immune protection against infection caused by V. alginolyticus than control VA0764. Finally, immune protection against infection caused by intracellular bacterium Edwardsiella tarda was assessed further using the top seven out of the 17 ompAs. This led to identification of three efficient polyvalent vaccines against infections caused by the extracellular bacterium V. alginolyticus and intracellular bacterium E. tarda. In addition, we sequenced genes for understanding mechanisms of the polyvalent vaccines, but association of immune protection with mutation of gene and amino acids is not determined. These results indicate that DNA shuffling is an efficient way to develop polyvalent vaccines against microbial infections.

Characterization of potential lytic bacteriophage against Vibrio alginolyticus and its therapeutic implications on biofilm dispersal

Vibrio alginolyticus is a leading cause of vibriosis, presenting opportunistic infections to humans associated with raw seafood contamination. At present, phage therapy that acts as an alternative sanitizing agent is explored for targeting V. alginolyticus. The study outcome revealed that the phage VP01 with its extreme lytic effect showed a high potential impact on the growth of V. alginolyticus as well as biofilm formation. Electron microscopy revealed the phage resemblance to Myoviridae, based on its morphology. Further study clarified that the phage VP01 possesses a broad host spectrum and amazing phage sensitivity at different pH, high thermal stability, and high burst size of 415 PFU/cell. In addition, the investigation of phage co-culturing against this pathogen resulted in a significant growth reduction even at less MOIs 0.1 and 1. These results suggest that the phage could be a promising candidate for the control of V. alginolyticus infections.