Fitness and transcriptomic analysis of pathogenic Vibrio parahaemolyticus in seawater at different shellfish harvesting temperatures

To better characterize the population dynamics of Vibrio parahaemolyticus (Vp) containing different virulence genes, two Vp strains were inoculated into seawater separately and incubated at temperatures (30 and 10 {degrees}C) mimicking summer and winter pre-harvest shellfish rearing seasons. The cellular responses of these two strains, one containing the thermostable direct hemolysin (tdh+) gene and the other one containing tdh-related hemolysin (trh+) gene, were studied at the transcriptomic level. Results showed that, at 30 {degrees}C, tdh+ and trh+ strains reached 6.77 {+/-} 0.20 and 6.14 {+/-} 0.07 Log CFU/ml respectively after 5 days. During this time, higher growth rate was observed in the tdh+ strain than the trh+ strain. When being kept at 10 {degrees}C, both Vp strains persisted at ca. 3.0 Log CFU/ml in seawater with no difference observed between them. Growth and persistence predictive models were then established based on the Baranyi equation. The goodness of fit scores ranged from 0.674 to 0.950. RNA sequencing results showed that downregulated central energy metabolism and weakened degradation of branched chain amino acid were observed only in trh+ strain not in tdh+ strain at 30 {degrees}C. This might be one reason for the lower growth rates of the trh+ strain at 30 {degrees}C. Histidine metabolism and biofilm formation pathways were significantly downregulated in both strains at 10 {degrees}C. No significant difference was observed for virulence-associated gene expression between 10 and 30 {degrees}C, regardless of the strains. SIGINIFICANCEGiven the involvement of Vp in a wide range of seafood outbreaks, a systematical characterization of Vp fitness and transcriptomic changes at temperatures of critical importance for seafood production and storage is needed. In this study, predictive models describing the behavior of Vp strains containing different virulence factors are established. While no difference was observed at the lower temperature (10 C), tdh+ strain had faster growth rate than the trh+ strain. Transcriptomic analysis showed that significantly higher number of genes were upregulated at 30 {degrees}C than 10 {degrees}C. Majority of differentially expressed genes of Vp at 30 {degrees}C were annotated to functional categories supporting cellular growth. At the lower temperature, the down regulation of the biofilm formation pathway and histidine metabolism indicates that the current practice of storing seafood at lower temperatures not only protect the seafood quality but also ensure the seafood safety.