Nile tilapia (Oreochromis niloticus) and European sea bass (Dicentrarchus labrax) are economically significant species in Mediterranean countries, serving essential roles in the aquaculture industry due to high market demand and nutritional value. They experience substantial losses from bacterial pathogens Vibrio anguillarum and Streptococcus iniae, particularly at the onset of the summer season. The immune mechanisms involved in fish infections by V. anguillarum and S. iniae remain poorly understood. This study investigated their impact through experiments with control and V. anguillarum- and S. iniae-infected groups for each species. Blood samples were collected at 1, 3, and 7 days post bacterial injection to assess biochemical and immunological parameters, including enzyme activities (AST and ALT), oxidative markers (SOD, GPX, CAT, and MDA), and leukocyte counts. Further analyses included phagocyte activity, lysozyme activity, IgM levels, and complement C3 and C4 levels. Muscle tissues were sampled at 1, 3, and 7 days post injection to assess mRNA expression levels of 18 immune-relevant genes. The focus was on cytokines and immune-related genes, including pro-inflammatory cytokines (TNF-α, TNF-β, IL-2, IL-6, IL-8, IL-12, and IFN-γ), major histocompatibility complex components (MHC-IIα and MHC-IIβ), cytokine receptors (CXCL-10 and CD4-L2), antimicrobial peptides (Pleurocidin and β-defensin), immune regulatory peptides (Thymosin β12, Leap 2, and Lysozyme g), and Galectins (Galectin-8 and Galectin-9). β-actin was used as the housekeeping gene for normalization. Significant species-specific responses were observed in N. Tilapia and E. Sea Bass when infected with V. anguillarum and S. iniae, highlighting differences in biochemical, immune, and gene expression profiles. Notably, in N. Tilapia, AST levels significantly increased by day 7 during S. iniae infection, reaching 45.00 ± 3.00 (p < 0.05), indicating late-stage acute stress or tissue damage. Conversely, E. Sea Bass exhibited a significant rise in ALT levels by day 7 in the S. iniae group, peaking at 33.5 ± 3.20 (p < 0.05), suggesting liver distress or a systemic inflammatory response. On the immunological front, N. Tilapia showed significant increases in respiratory burst activity on day 1 for both pathogens, with values of 0.28 ± 0.03 for V. anguillarum and 0.25 ± 0.02 for S. iniae (p < 0.05), indicating robust initial immune activation. Finally, the gene expression analysis revealed a pronounced peak of TNF-α in E. Sea Bass by day 7 post V. anguillarum infection with a fold change of 6.120, suggesting a strong species-specific pro-inflammatory response strategy. Understanding these responses provides critical insights for enhancing disease management and productivity in aquaculture operations.
