Genomic analysis of a new type VI secretion system in Vibrio parahaemolyticus and its implication for environmental adaptation in shrimp ponds.

Abstract

Type VI secretion system (T6SS) in Vibrio spp., is often used to kill heteroclonal neighbors by direct injection of toxic effectors, but its strategies in the aquacultural environments receives limited attention. In this study, we conducted genomic analysis for a T6SS-harboring plasmid in V. parahaemolyticus strain VP157. Co-culture assays were further conducted to verify its antibacterial function. Results showed that strain VP157 harbored a 132 kb plasmid pVP157-1 which consist of two fragments: a 87.8 kb fragment identical with plasmid pTJ114-1 and a 44.2 kb T6SS1 operon with only 4% DNA identity to T6SS1 in V. parahaemolyticus reference genome. Gene-by-gene analysis of six genes representing core T6SS component suggested that each gene has distinct evolutionary origins. In vitro experimental evolution revealed that pVP157-1 can excise from the VP157 genome with an excision rate of 4%. Co-culture assay suggested that strain VP157 had significantly higher antibacterial activity against Bacillus pumilus and V. chorelae relative to strain without pVP157-1(VP157∆T6SS1). In contrast, a rapid decline was observed for the proportion of VP157∆T6SS1 in a mock microbial community, which decreased from 10.7% to 2.1% in five days. Results highlighted that the acquisition of T6SS fostered the fitness of V. parahaemolyticus in a complex environment.