Is Shewanella oncorhynchi a fish health threat? Possible genetic background of pathogenicity and common carp challenge.

Introduction: The Shewanella genus is widely distributed in aquatic environments and occurs in different niches with a wide range of temperatures and salinities. It includes a novel species, S. oncorhynchi, described as recently as 2022, causing lens atrophy, inappetence and growth retardation in rainbow trout (Oncorhynchus mykiss, Walbaum 1792). Shewanella oncorhynchi also occurs in common carp (Cyprinus carpio L.), but its potential pathogenicity in this species is unclear.

Material and methods: In this study, the pathogenicity of three well-characterised S. oncorhynchi strains was assessed in vivo in experimental infection of common carp and in vitro by estimation of their cytotoxicity to an epithelioma papulosum cyprini cell line. The strains' characterisation involved whole-genome sequencing to identify possible virulence genes.

Results: Our study proved the pathogenicity of S. oncorhynchi to common carp and the bacterium's cytotoxicity to epithelioma papulosum cyprini cells. Swollen abdomens, lens opacity, areas of discoloration and skin lesions were recorded in infected common carp. However, the ability to cause disease symptoms and mortality depended on the strain.

Conclusion: The study showed the potential roles of the quorum-sensing system, type IV pili, fimbriae, stress survival, iron metabolism and secretion system genes in the virulence of S. oncorhynchi. Of these, the vscC2, vscN2, vscR2, vscS2 and vscU2 type III secretion system genes potentially and probably enhance the bacterium's ability to cause lesions in common carp.