Virulence of the extracellular products of highly pathogenic strains of Photobacterium damselae subsp. damselae

Abstract:

[Objective] Photobacterium damselae subsp. damselae (PDD) is a major pathogen which causes serious diseases in marine organisms. This study focused on two highly pathogenic PDD strains (PDD1605 and PDD1608) with strong hemolysis and phospholipase activities, which were isolated from maricultured fish in China. We analyzed the virulence and cytotoxicity of the extracellular products (ECPs) derived from the two PDD strains and their correlations with the strain pathogenicity. [Methods] The hemolysis and phospholipase activities of ECPs from PDD1605 and PDD1608 were measured on agar plates. The pathogenicity of the two live strains and their ECPs to Sebastes schlegeli were detected by artificial challenge test. Meanwhile, we prepared histopathological sections to observe the pathological damage of organs and tissues. Furthermore, we added the ECPs to the media of human embryonic kidney cell line HEK293T and mouse cardiac fibroblasts (MCFS) to study the toxicity of ECPs to mammalian cells.[Results] The live PDD1605 and PDD1608 strains showed strong pathogenicity to S. schlegeli. All the fish exposed to the strains at the concentration of 5×10⁸ CFU/mL died within 24 h. The ECPs of PDD1605 and PDD1608 also showed strong pathogenicity to S. schlegeli. The death curve trend was similar between ECPs and live strains. The ECPs caused serious tissue damage to S. schlegeli, which mainly included disintegration of intestinal mucosa, blood cell and inflammatory cell infiltration in liver, hemosiderosis in spleen, and inflammatory cell infiltration in myocardium. Additionally, the ECPs of the two PDD strains had strong hemolysis and phospholipase activities in vitro, which was basically consistent with the virulence characteristics of live strains. Moreover, they had cytotoxicity to HEK293T and MCFS cells, reducing the cell volume in vitro and causing degenerative changes such as round cell clusters.[Conclusion] TheECPs and live strains of PDD1605 and PDD1608 have highly consistent virulence and cytotoxicity. Our findings support that ECPs are the key pathogenic factor of highly pathogenic PDD strains.