Delineating the key virulence factors and intra-species divergence of Vibrio harveyi by the use of whole genome sequencing.

Vibrio harveyi is one of the major pathogens in aquaculture. To identify the key virulence factors affecting pathogenesis of V. harveyi towards fish, a field investigation was conducted for three representative fish farms infected with V. harveyi. Multilocus sequence typing (MLST) and whole-genome sequencing were conducted to delineate phylogenetic relationship and genetic divergence of V. harveyi. A total of 25 V. harveyi strains were isolated from the diseased fish and groundwater, which can be subtyped into 12 sequence types (STs) by MLST. Five virulence genes including mshB, pilA, hutR, ureB and ureG were variably presented in sequenced strains. The virulence gene profiles strongly correlated with the distinct pathogenicity of V. harveyi strains, in which a strain harbored all five genes exhibited the highest virulence towards fish. Phenotype assay confirmed that reduced virulence correlated with decreased motility and biofilm formation ability. Additionally, three types of type VI secretion system (T6SS), namely T6SS1, T6SS2 and T6SS3 were identified in V. harveyi strains, which can be classified into six, four and 12 subtypes, respectively. In conclusion, results indicated that the virulence level of V. harveyi is mainly determined by above virulence genes, which may play vital roles in environmental adaptation for V. harveyi.