Vibrio parahaemolyticus Down-Regulates the Intracellular c-di-GMP Level to Promote Swarming Motility by Sensing Surface.

Vibrio parahaemolyticus, a significant food-borne pathogen that causes economic and public health problems worldwide, can produce two types of flagella, the single polar flagellum responsible for swimming in a liquid environment and the lateral flagella (laf) that enable the bacteria to swarm on the tops of solid surfaces. The polar flagellar genes are expressed either in liquid or on a surface, however, laf genes would only be activated by surface sensing. In this study, the molecular mechanism of surface sensing activating laf gene expression in V. parahaemolyticus was investigated. We found that the c-di-GMP concentration for liquid-grown cells was higher than the concentration for surface-grown cells and laf gene expression could be activated without touching surface if the intracellular level of c-di-GMP was decreased by overexpressing the scrABC operon in the wild-type strain. Surface sensing inhibits the transcription of those c-di-GMP-metabolizing enzymes which could negatively regulate swarming, and enhances those which could positively regulate swarming. Surface sensing also enhances the activity of quorum sensing (QS) of V. parahaemolyticus which plays an important role in regulating the transcription of components of the c-di-GMP network. Combined, the data indicate that V. parahaemolyticus enhances QS by surface sensing to down-regulate the intracellular level of c-di-GMP which results in producing the lateral flagella and swarming on a surface. What we found in this study suggests an important signal transduction pathway of regulating swarming motility by surface sensing in V. parahaemolyticus.