Virulence, genomic features, and antimicrobial resistance of a highly virulent Streptococcus iniae ST4 from diseased yellow catfish.

Streptococcosis is an emerging bacterial disease in freshwater aquaculture that poses a substantial threat to animal health and production sustainability. In this study, the etiological agent responsible for a "whirling disease" outbreak in yellow catfish (Pelteobagrus fulvidraco) was isolated and comprehensively characterized. A dominant bacterial strain, designated as HSY24518-1S, was identified as Streptococcus iniae, physiological, and biochemical characteristics, as well as lctO and 16S rRNA gene sequence analyses. Multilocus sequence typing (MLST) assigned the isolate to sequence type 4 (ST4). Whole-genome sequencing revealed a draft genome of 2.05 Mb (GenBank accession JBVLZU000000000), containing 219 putative virulence-related genes and 161 antimicrobial resistance-associated genes. Histopathological examination of experimentally infected fish revealed severe hemorrhagic and degenerative lesions, predominantly in the brain, liver, and spleen. PCR assays confirmed the presence of seven key virulence genes (simA, scpI, pgm, cpsD, pdi, sagA, and cfi). Challenge experiments yielded 100% mortality at concentrations from 1.23 × 104 to 1.23 × 108 CFU/mL, with a median lethal dose (LD50) of 2.48 × 103 CFU/mL. Antimicrobial susceptibility testing showed that the strain was susceptible to 14 antibiotics, including cefoperazone, amoxicillin, and doxycycline, but resistant to gentamicin, kanamycin, streptomycin, and neomycin. These findings provide detailed insights into the genomic characteristics, virulence potential, and antimicrobial resistance profile of a highly virulent S. iniae strain, thereby supporting improved diagnosis, treatment, and control of streptococcosis in cultured fish.