Genomic characterization of streptococcus agalactiae and streptococcus suis isolated from diseased snakeskin gourami (trichopodus pectoralis)

This study investigates severe, recurrent mortality outbreaks in farmed snakeskin gourami (Trichopodus pectoralis) in central Thailand between 2021 and 2023. Characterized by exophthalmos and systemic hemorrhage, these incidents prompted an investigation to identify causative agents, assess antimicrobial resistance (AMR), and explore genomic adaptation. Bacterial isolates from moribund fish in Samut Sakhon province were identified via MALDI-TOF MS and 16S rRNA sequencing, followed by whole-genome sequencing (Illumina HiSeq) and comparative pan-genomic analysis. Two primary pathogens, Streptococcus agalactiae and Streptococcus suis, were confirmed as causative agents of acute bacterial septicemia, occurring in both single and co-infections. Phylogenetic analysis revealed that S. suis isolates formed a distinct, homogeneous sub-lineage unique to snakeskin gourami, genetically separate from terrestrial mammalian strains, while S. agalactiae clustered with the piscine serotype VII. Phenotypically, S. suis was susceptible to all antibiotics, whereas S. agalactiae exhibited oxytetracycline intermediate resistance. Genomic analysis confirmed this contrast, identifying the tet(M) gene on T4SS-type integrative and conjugative elements (ICEs) in S. agalactiae, while S. suis lacked significant resistance determinants. Pan-genomic profiling indicated divergent evolutionary strategies: the fish-associated S. suis exhibited an expanded accessory genome enriched for carbohydrate metabolism, oxidative stress tolerance, and defense mechanisms, suggesting specific adaptation to the aquatic environment. Conversely, S. agalactiae displayed high genomic plasticity driven by mobile elements. These findings provide the first genomic evidence of S. suis adaptation to a fish host. The study underscores the evolutionary convergence of terrestrial and aquatic pathogens, highlighting the urgent need for genomic surveillance to safeguard animal and public health.