Expression and Immune Response Profiles in Nile Tilapia () and European Sea Bass () During Pathogen Challenge and Infection.

Nile tilapia () and European sea bass () 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 and , particularly at the onset of the summer season. The immune mechanisms involved in fish infections by and remain poorly understood. This study investigated their impact through experiments with control and - and -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 (, , , , , , and ), major histocompatibility complex components ( and ), cytokine receptors ( and ), antimicrobial peptides ( and ), immune regulatory peptides (, , and ), and Galectins ( and ). was used as the housekeeping gene for normalization. Significant species-specific responses were observed in N. Tilapia and E. Sea Bass when infected with and , highlighting differences in biochemical, immune, and gene expression profiles. Notably, in N. Tilapia, AST levels significantly increased by day 7 during infection, reaching 45.00 ± 3.00 ( < 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 group, peaking at 33.5 ± 3.20 ( < 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 and 0.25 ± 0.02 for ( < 0.05), indicating robust initial immune activation. Finally, the gene expression analysis revealed a pronounced peak of in E. Sea Bass by day 7 post 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.