Comparative analysis of Streptococcus agalactiae S03 and S07 isolated from Schizothorax spp. with different antibiotic resistance and virulence

Streptococcus agalactiae (GBS) is an important Gram-positive pathogen of fish aquaculture worldwide. In this study, we performed a comparative analysis of GBS S03 (serotype III) and S07 (serotype Ia) isolated from Schizothorax spp.. We explored the association between phenotypic antibiotic resistance, virulence, and genomic characteristics. Antimicrobial sensitivity tests on 12 common antibiotics using the disc diffusion method revealed that the GBS S03 resisted seven antibiotics, while GBS S07 showed sensitivity to eleven antibiotics. Pathogenicity analysis demonstrated greater virulence of GBS S07 than S03. Then, the occurrence of antibiotic resistance and virulence genes were identified using the whole-genome sequence (WGS) of GBS S03 and GBS S07. Besides just GBS S03 had gyrA and parC gene mutations, there were mefE, tetO, lnuB, lsaE, APH3’, and sat-4 resistance genes present only in GBS S03 genome. In addition to 51 virulence genes in both GBS S03 and S07 genomes, GBS S07 carried virulence genes associated with invasion, such as SAN_1519, rfbA and cylE genes. There was complete concordance between genotypic evidence and phenotypic characteristics. Virulence factors and phylogenetic analysis showed that GBS S03 and human sources shared an extremely close evolutionary relationship. Our findings also provide important proof for using WGS as an effective tool for phenotypic predictions of GBS.