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Mud crab (Scylla paramamosain) is an important economic species in China. Vibrio parahaemolyticus infection have caused a great economic loss in mud crab farming. The mechanism involved in the immune responses of mud crab to V. parahaemolyticus is unclear. In this study, the physiological and immune response to V. parahaemolyticus infection were investigated in S. paramamosain. The results showed that V. parahaemolyticus infection decreased total hemocyte counts, led to cytological damage, and caused high mortality. Transcriptome analysis showed that 1327 differentially expressed genes (DEGs), including 809 up-regulated and 518 down-regulated ones, were obtained after V. parahaemolyticus challenge. These DEGs were mainly involved in the immune response and infectious disease. Additionally, transcriptome analysis revealed that Toll, immune deficiency (IMD), and prophenoloxidase signalling pathways played essential roles in antibacterial immunity against V. parahaemolyticus infection in mud crab. • V. parahaemolyticus led to cytological damage, and caused high mortality. • 1327 DEGs were obtained after V. parahaemolyticus challenge. • Toll, IMD, and prophenoloxidase signalling pathways played important roles in response to V. parahaemolyticus infection. [ABSTRACT FROM AUTHOR]

To study the relationship between environmental and clinical populations of Vibrio parahaemolyticus, we collected in total 86 isolates from Southern China during one and a half years. Sixty-eight isolates were recovered from aquaculture ponds, a seafood market, and restaurants, and 18 isolates were recovered from clinical samples. Virulence gene analysis revealed that 25 isolates (14 clinical and 11 environmental) tested positive for tdh, but only 4 carried trh. Interestingly, none of the tdh(+) environmental isolates was recovered from ponds. Both environmental and clinical tdh(+) isolates, except for one clinical isolate, harbor type III secretion system 2α (T3SS2α) and T3SS2β-related genes, including vopB2α, which was previously suggested to be absent from environmental strains. More than 70% of clinical isolates carried the pandemic marker of new toxRS (GS-PCR(+)), which was not present in the environmental isolates. Pulsed-field gel electrophoresis and multilocus sequence typing analysis showed a high degree of genetic diversity within the environmental isolates. In contrast, the clinical population formed a tight cluster that differed from the environmental isolates. These findings suggest that the pandemic strains of V. parahaemolyticus may not directly originate from marine animals. Rather the environments where they are maintained could serve as reservoirs for toxigenic, but not pandemic strains. These environments provide an ideal place for generation of new toxigenic strains through DNA exchange, which was revealed by extensive recombination events in recA sequences of the environmental isolates.