Temperature and salinity modulate virulence and PirA gene expression of Vibrio parahaemolyticus, the causative agent of AHPND

Global cultured shrimp production has sustained considerable losses due to acute hepatopancreatic necrosis disease (AHPND) caused by some strains of Vibrio that secrete PirAB binary toxin. Previous experiments have revealed changes in mortality due to AHPND under variable environmental conditions. In order to determine whether and how environmental factors affect the pathogenicity of a strain of Vibrio parahaemolyticus that causes AHPND (Vp_AHPND), a Vp_AHPND strain was grown at different temperatures (28, 30, and 33 °C) and proportions of NaCl (2.0, 3.0, and 3.5%). PirA gene expression was assessed in these bacterial cultures, and bioassays by immersion challenge were performed at 20, 30, and 35 practical salinity units (PSU). The results showed that salinities and temperatures tested do not inhibit Vp_AHPND growth. However, maximum PirA gene expression at 3.5, 3.0, and 2.0% NaCl reached 21.1, 13.0, and 0.03 millions of copies per ng of RNA, respectively, and at 28, 30, and 33 °C reached 48.5, 21.1, and 11.3 millions of copies per ng of RNA, respectively. Furthermore, the mortality rate in the bioassay challenges corresponded proportionally to the expression of the PirA gene, reaching 83.3, 46.6, and 0 mortality at 35, 30, and 20 PSU, respectively; and 90.0, 63.3, and 43.3 mortality at 28, 30, and 33 °C, respectively. These results indicate that NaCl concentration and temperature modify the expression of the PirA gene and regulate Vp_AHPND virulence. Since the virulence of Vp_AHPND is sensitive to environmental factors, it may be a potent target for therapeutic intervention in seasonal bacterial disease.