Comparative Proteomic Analysis of Streptococcus agalactiae Isolates with Distinct Virulence Phenotypes

Streptococcus agalactiae causes important disease losses in aquaculture, yet isolates of this species can differ markedly in pathogenicity. The molecular basis of these isolate-level differences remains incompletely understood. Here, we compared two fish-derived S. agalactiae isolates, MML SA-006 and MML SA-010, by assessing their effects in giant hybrid grouper and performing a comparative proteomic analysis using two-dimensional gel electrophoresis (2-DE) and liquid chromatography–tandem mass spectrometry (LC-MS/MS). Under the challenge conditions used, MML SA-006 produced a more severe disease outcome than MML SA-010. Analysis of the selected gel spots identified 24 spots representing 19 distinct proteins. These proteins were classified into four broad functional groups: protein translation, folding, and stress response; RNA modification, transcriptional regulation, and nucleic acid metabolism; carbohydrate and energy metabolism; and amino acid, nucleotide, and redox metabolism. The findings describe differences in protein abundance between the two isolates and provide a basis for further investigation of their potential biological significance. Further functional studies are needed to determine whether these proteins are involved in the observed differences in disease outcomes.