A comprehensive perspective on a Vagococcus salmoninarum outbreak in rainbow trout broodstock

Abstract

Streptococcus species are of major concern for both warm-water and cold-water salmonid species, and have zoonotic importance. While Lactococcus garvieae, associated with warm-water streptococcosis, causes thousands of tons of mortalities, Vagococcus salmoninarum, the causative agent of cold-water streptococcosis, leads to chronic disease and devastating mortality in broodstock. In the present study, five bacterial samplings were performed from outbreaks that occurred between winter 2019 and summer 2020 in rainbow trout broodstock. The isolates were comprehensively characterized by phenotypic tests, using the API 20 Strept, Rapid ID 32 Strept and BIOLOG GENIII test kits. The antimicrobial susceptibilities of the strains were compared by the disk diffusion method, according to the guidelines of the Clinical & Laboratory Standards Institute (CLSI), additionally on commonly used media such as mueller-hinton agar and tryptic soy agar. Following a genomic approach, we analyzed the VS-1 strain by whole-genome sequencing. Virulence genes and correlation data were supported with the histopathological examination of the affected organs and tissues. The genome of the VS-1 strain included 69 virulence genes and 31 antimicrobial resistance genes. This study is one of the most comprehensive researches on V. salmoninarum, and elucidates the phenotypic, genotypic, antimicrobial resistance and virulence characteristics of this species.