Characterization of the rpoS gene and its role in the survival of Vibrio parahaemolyticus under environmental stresses

Vibrio parahaemolyticus is a foodborne pathogen isolated from marine environment and seafoods. A variety of environmental stresses influence the physiology and life cycle of V. parahaemolyticus in its natural environment, and it has to overcome these stresses in order to persist in its habitat. An important bacterial mechanism on exposure to adverse environmental conditions is modulation of expression of the alternate sigma factor σS or RpoS, encoded by the gene rpoS, which in turn activates genes needed for survival under stress conditions. Although the role played by rpoS in stress survival has been studied in a number of bacteria, no information is available on the genetic mechanisms contributing to stress survival in V. parahaemolyticus . The present study investigated the growth characteristics of V. parahaemolyticus on fish fillets stored at low temperatures as well as the role of rpoS in survival of V. parahaemolyticus under various stresses. V. parahaemolyticus survived on fish fillets stored at refrigeration temperatures, while on frozen fillets, survival was dependent on the level of contamination. An rpoS mutant strain of V. parahaemolyticus was significantly impaired in its ability to survive stress conditions, including oxidative, osmotic and heat stresses. Two-dimensional polyacrylamide gel electrophoresis of total proteins from V. parahaemolyticus revealed that the expression of 10 proteins each on exposure to salt stress and heat stress was significantly reduced in the mutant, in comparison to the wild type. RT-PCR experiments also revealed that expression of the rpoS gene was lower in the mutant compared to the wild following exposure to heat stress and salt stress. Survival of the mutant strain in artificial seawater and fish homogenate was found to be significantly lower than that of the wild strain. Results of this study indicate that rpoS is involved in the adaptation of V. parahaemolyticus to adverse environmental stresses. Moreover, it also helps V. parahaemolyticus to tide over osmotic stress and cold stress in seawater and fish. Thus, rpoS potentially plays a critical role in the persistence of V. parahaemolyticus in its natural habitat, thereby leading to contamination of seafoods.