Effects of pH and temperature on the virulence of Vibrio Parahaemolyticus

Abstract

Vibrio parahaemolyticus is a Gram-negative bacterium that can cause Early Mortality Syndrome (EMS), now named Acute Hepatopancreatic Necrosis Disease (AHPND) in shrimp. AHPND resulted in serious damage to the shrimp particularly white-leg shrimp culture around the world including Viet Nam. This study aimed to investigate the effect of pH and temperature on the virulence of V. parahaemolyticus represented via production of various exoenzymes. V. parahaemolyticus XN9 was cultured in BHI medium of different pH (7.5, 8, 8.5, 9) and temperature (25oC, 30oC, 35oC) with pH 8.5 and 30oC as standard condition then six extracellular enzymes including caseinase, hemolysin, lecithinase, chitinase, gelatinase and lipase, were assessed using agar-based method. As a result, there were significant differences in enzyme activities of V. parahaemolyticus under different pH and temperature. We found that when pH increased from 7.5 to 9, activity of alpha hemolysin decreased slightly from 7.67 ± 0.64 mm to 6.07 ± 0.49 mm. In addition, caseinase activity was also reduced significantly (by 88%, from 3.55 ± 0.25 to 0.41 ± 0.08 mm), whereas gelatinase activity increased markedly from negative result to 1.38 ± 0.17 mm. At pH 8.5, chitinase and lecithinase showed strongest extracellular enzymatic activity which were 1.51 ± 0.09 and 2.96 ± 0.13 mm, respectively. Under the effect of temperature, most of virulence factor tests highest activity at 30oC such as lecithinase, hemolysin, chitinase in which the degree of enzyme influence was up to 2.96 ± 0.13, 7.32 ± 0.65, 1.51 ± 0.1 mm, respectively. Besides, the ability to produce caseinase and gelatinase of V. parahaemolyticus also increased by 28% and 59% following the rise of temperature from 25oC to 35oC. Our study suggested that warm temperature (30oC) and alkaline condition (pH 8.5) can increase the virulence of V. parahaemolyticus, thus promoting the emergence of AHPND. Keywords: Vibrio parahaemolyticus, Early Mortality Syndrome (EMS), Acute Hepatopancreatic Necrosis Disease (AHPND), virulence factor, shrimp, enzyme activity