Effects of environmental factors on growth and VTc toxin biosynthesis in Vibrio parahaemolyticus associated with a shrimp disease

Highly lethal Vibrio disease (HLVD), also known as translucent post-larval disease, emerged in late 2019 and has led to acute, mass mortality in Penaeus vannamei post-larvae. HLVD is caused by Vibrio strains that produce VTc toxins. In this study, we examined how temperature, pH, salinity, and nutrient availability influence growth and VTc toxin production in the HLVD-causing Vibrio parahaemolyticus strain vp-HL-202005. The bacterium grew well in LB medium at 25–35°C, pH 6–9, and salinities of 5–30 ‰. Whereas the optimal conditions for VTc toxin production were 25–30℃, pH 6–9, and salinity 5–30 ‰. Reducing nutrient supply markedly suppressed both growth and VTc toxin synthesis. Under optimal culture conditions, biomass plateaued at 24 h, whereas VTc toxins continued to accumulate. Because shrimp-pond salinity, pH and nutrient levels usually fall within this favorable range, we conducted immersion-challenge trials to determine whether temperature shifts trigger HLVD outbreaks. When post-larvae were held at 35°C, vp-HL-202005 proliferation was suppressed and cumulative mortality remained below 20 %. Lowering the temperature to 30°C alone did not significantly affect bacterial proliferation or shrimp mortality. However, when the water was simultaneously replaced as the temperature dropped, vp-HL-202005 proliferated rapidly and killed > 90 % of the animals within 3 days. Thus, combined changes in temperature and water conditions may be key triggers of HLVD outbreaks in shrimp farms. • We determine the optimal conditions for the growth and VTc toxin production in a HLVD-causing Vibrio parahaemolyticus strain. • The growth and VTc toxin production in the HLVD-causing strain are suppressed at high temperature. • The changes in temperature and water environment may be the key factors that induce HLVD outbreaks in shrimp farms.