Comprehensive analysis of dominant Streptococcus iniae strains from diseased rockfish (Sebastes schlegelii) in South Korea: phenotypic, genetic, and pathogenic profiles

Abstract

Streptococcus iniae is a pathogen that affects fish, causing considerable economic losses in aquaculture worldwide. From 2002 to 2021, an analysis of 57 strains causing streptococcosis in cultured rockfish (Sebastes schlegelii) in South Korea, using 16S rRNA sequencing, revealed that 82.46% of the cases were caused by S. iniae. To enhance our understanding of S. iniae infection, we investigated its phenotypic, genotypic, and pathogenic profiles. Biochemical characteristics analysed using the API 20 strep kit and haemolytic activities (α-haemolysis, 15.6%; β-haemolysis, 81.3%; γ-haemolysis, 3.1%) varied based on the isolation area. Genotypic profiling was performed using multilocus sequence analysis (MLSA) of ten genes: cpsD, mutS, rpoB, glnA, gyrB, prkC, tkt, pheT, arcC, and groEL. Phylogenetic tree identified two clades, with 90.6% of isolates belonging to MLSA-clade 1. Pathogenicity experiments using juvenile rockfish (average weight, 14.94 g) demonstrated that MLSA-clade 1 isolates caused significantly higher mortality than MLSA-clade 2 isolates following intraperitoneal injection. The lethal dose (LD₅₀) for S. iniae isolates 17FBSIn0006, 23FBStr0601, 23FBStr0602, and 23FBStr0603 were 4.58 CFU/fish, 4.28 CFU/fish, 4.71 CFU/fish, and 5.6 × 10⁵ CFU/fish, respectively. Additionally, Lactococcus garvieae 1708–35 (LD₅₀: 1.57 × 10⁷ CFU/fish) and S. parauberis Namhae17 (LD₅₀: 3.19 × 10⁶ CFU/fish) exhibited relatively low pathogenicity in rockfish. Sequence alignment of cpsD-encoded capsular polysaccharide D protein of S. iniae isolates revealed differences at three amino acid positions (123, 129, and 159); these were assumed to be related to the pathogenic diversities of S. iniae. Our findings provide valuable insights into streptococcosis in rockfish.