Identification, molecular typing, antimicrobial susceptibility, and experimental pathogenicity of Streptococcus agalactiae isolated from Tilapia.

Streptococcosis caused by Streptococcus agalactiae is widespread in the world, especially in south Asia and southeast Asia. In this study, an S. agalactiae isolate collected from diseased fish in India was molecularly confirmed through GAPDH-targeted PCR, and further validated by sequencing. Genotyping revealed distinct genomic relationships among regional isolates; pulsed-field gel electrophoresis identified three pulsotypes with the Indian isolate clustering closely with two Taiwanese isolates under both ApaI and SmaI digestion, while ERIC-PCR grouped the Indian strain with a Taiwanese strain at > 92% similarity. Antibiotic susceptibility test showed that the Indian isolate was resistant to tetracycline, in contrast to the control strain. Pathogenicity assays determined an LD50 of 1.7 × 10⁵ CFU/ml, and infected fish displayed typical clinical signs consistent with streptococcosis. Pro-inflammatory cytokines (IL-1β, IL-8, IFN-γ) were significantly upregulated in head kidney and spleen tissues at 48 h post infection, although IL-8 and IL-1β showed earlier expression in the spleen. These results characterize the molecular identity, virulence, and immunopathological profile of an Indian S. agalactiae strain, providing critical insights for disease surveillance in tilapia aquaculture.