Genomic and Phenotypic Characterisation of Streptococcus parauberis Isolated From Starry Flounder ( Platichthys stellatus ): First Whole‐Genome Analysis, Pathogenicity Assessment, and Antimicrobial Resistance Profiling

Streptococcus parauberis is recognised as a major pathogen in marine aquaculture species. This pathogen has been reported in Platichthys stellatus aquaculture farms in South Korea. To investigate the causative agent responsible for mass mortality, monthly disease monitoring was conducted at three aquaculture farms in Pohang. A total of 31 S. parauberis isolates were identified using Spa primers. Additionally, to confirm the pathogenicity of S. parauberis , starry flounder were intraperitoneally injected with strain AD112410, which resulted in a dose‐dependent mortality pattern. Whole genome sequencing was conducted on strain AD112410 isolated from starry flounder and compared with that of other S. parauberis strains isolated from olive flounder. All S. parauberis isolates were classified as serotype Ia and exhibited γ‐hemolysis. PCR detection revealed the presence of the tet (S) resistance gene in all farms, while erm (B) was absent only in farm C. Virulence genes ( gapC and hasC ) were found in all farms. All isolates showed resistance to tetracycline and oxytetracycline, while susceptibility to erythromycin and clindamycin depending on the presence of erm (B). These findings not only demonstrate an association between S. parauberis and mass mortality in starry flounder, but also indicate a correlation among serotype, antibiotic resistance genes, and antibiotic susceptibility.