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

ABSTRACT 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 (10°C and 30°C) mimicking winter and summer pre-harvest shellfish rearing seasons. The cellular responses of two Vp 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°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, a higher growth rate was observed in the tdh+ strain than in the trh+ strain. When being kept at 10°C, both Vp strains persisted at ca. 3.0 log CFU/mL in seawater with no difference observed between them. Survival and growth 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 the trh+ strain not in the tdh+ strain at 30°C. This might be one reason for the lower growth rates of the trh+ 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°C and 30°C, regardless of the strains. IMPORTANCE Given the involvement of Vibrio parahaemolyticus (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, one of each virulent Vp strain (tdh+ and trh+) was tested. While no difference in survival behavior of the two virulent strains was observed at 10°C, the tdh+ strain had a faster growth rate than the trh+ strain at 30°C. Transcriptomic analysis showed that a significantly higher number of genes were upregulated at 30°C than at 10°C. The majority of differentially expressed genes of Vp at 30°C were annotated to functional categories supporting cellular growth. At 10°C, the downregulation of the biofilm formation and histidine metabolism indicates that the current practice of storing seafood at low temperatures not only protects seafood quality but also ensures seafood safety. Given the involvement of Vibrio parahaemolyticus (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, one of each virulent Vp strain (tdh+ and trh+) was tested. While no difference in survival behavior of the two virulent strains was observed at 10°C, the tdh+ strain had a faster growth rate than the trh+ strain at 30°C. Transcriptomic analysis showed that a significantly higher number of genes were upregulated at 30°C than at 10°C. The majority of differentially expressed genes of Vp at 30°C were annotated to functional categories supporting cellular growth. At 10°C, the downregulation of the biofilm formation and histidine metabolism indicates that the current practice of storing seafood at low temperatures not only protects seafood quality but also ensures seafood safety.