A Vibrio owensii strain as the causative agent of AHPND in cultured shrimp, Litopenaeus vannamei.

The causative agent of shrimp AHPND was identified as specific Vibrio parahaemolyticus strains, which harbor a virulent plasmid that contains the toxic genes pirA and B (pirAB). Herein, a Vibrio bacterium was isolated from shrimp in Shanghai. This bacterium was identified as Vibrio owensii using 16S rRNA gene phylogeny, whole genome sequencing and comparative analysis. The V. owensii cells are rod-shape (1.86 ± 0.15 µm) with a single polar flagellum (4 µm). In addition, V. owensii form mauve colonies with jagged edges on CHROMagar plates. The pirAB genes on the plasmid revealed 100% sequence similarity to that of AHPND V. parahaemolyticus, and the encoded proteins were detected in the culture media. Subculture of V. owensii showed that the pirAB genes are unstable, and their loss rate is approximately 22% and reaches a dynamic equilibrium after the fifth generation. Upon immersion bioassay, the cumulative mortality of V. owensii (pirAB+)-infected shrimp was up to 100% within 4 days, and typical AHPND clinical signs were observed. Approximately 105 CFU/hepatopancreas of V. owensii cells were observed in the pirAB+-infected shrimp based on both culture-dependent and -independent assay. Our results indicate that the expression of pirAB in the V. owensii strain is responsible for AHPND.