Vibrio parahaemolyticus becomes highly virulent by producing Tc toxins

Highly lethal Vibrio disease (HLVD) is a newly emerging disease caused by virulent strains of Vibrio parahaemolyticus (VpHLVD), which leads to acute and mass mortality in shrimp post-larvae. Previous research suggests that the pathogenicity of VpHLVD may be associated with an unknown toxin(s). To screen for the HLVD-related bacterial toxin(s), we compared the protein profiles of VpHLVD and non-HLVD-causing Vibrio strains and identified two VpHLVD-specific protein bands of ∼300 kDa and ∼ 200 kDa. They were co-purified from the bacterial lysate with a protein of ∼100 kDa by ion-exchange chromatography. Immersion challenge using this protein fraction produced the HLVD pathology in Penaeus vannamei. LC-MS/MS analysis indicated that the three major bands in this toxic fraction are Tc toxin subunits, TcA, TcB, and TcC. The genome of VpHLVD strain vp-HL-202005 carries two sets of plasmid-encoded Tc toxins, which contain three distinct TcC subunits. The specific expression of these Tc toxin subunits in VpHLVD strains was confirmed by RT-PCR and Western blotting. Removal of the toxin-encoding plasmids completely abolished the HLVD-causing ability of VpHLVD. These data demonstrate that HLVD is caused by the Tc toxins produced by VpHLVD.