Fitness and transcriptomic analysis of pathogenicVibrio parahaemolyticusin seawater at different shellfish harvesting temperatures

Abstract

ABSTRACTTo better characterize the population dynamics ofVibrio parahaemolyticus(Vp) containing different virulence genes, twoVpstrains were inoculated into seawater separately and incubated at temperatures (30 and 10 °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 containingtdh-related hemolysin (trh+) gene, were studied at the transcriptomic level. Results showed that, at 30 °C,tdh+ andtrh+ 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 thetdh+ strain than thetrh+ strain. When being kept at 10 °C, bothVpstrains 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 intrh+ strain not intdh+ strain at 30 °C. This might be one reason for the lower growth rates of thetrh+ strain at 30 °C. Histidine metabolism and biofilm formation pathways were significantly downregulated in both strains at 10 °C. No significant difference was observed for virulence-associated gene expression between 10 and 30 °C, regardless of the strains.SIGINIFICANCEGiven the involvement ofVpin a wide range of seafood outbreaks, a systematical characterization ofVpfitness and transcriptomic changes at temperatures of critical importance for seafood production and storage is needed. In this study, predictive models describing the behavior ofVpstrains containing different virulence factors are established. While no difference was observed at the lower temperature (10 C),tdh+ strain had faster growth rate than thetrh+ strain. Transcriptomic analysis showed that significantly higher number of genes were upregulated at 30 °C than 10 °C. Majority of differentially expressed genes ofVpat 30 °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.