Dietary Xylooligosaccharide Improves Growth, Immune Response, and Disease Tolerance of Olive Flounder (Paralichthys olivaceus)

The impacts of dietary xylooligosaccharide (XOS) were explored on growth, blood biochemistry, immune response, and resilience of olive flounder (Paralichthys olivaceus) against Edwardsiella tarda. Three replicate groups of fish (47.2 ± 0.41 g) were fed four diets incorporating various doses of XOS [0% (control), 1%, 2%, and 3%] to apparent satiety for eight weeks. Dietary inclusion of XOS improved growth performance and increased hepatosomatic and viscerosomatic indices. Serum alanine aminotransferase activity decreased with XOS supplementation, indicating improved liver status. Key innate immune parameters, including serum lysozyme, antiprotease, and alternative complement (ACH50) activities, were enhanced in XOS-fed fish. Skin mucus protein content also increased, whereas serum myeloperoxidase and glutathione peroxidase activities, as well as skin mucus antiprotease and protease activities, remained unchanged. XOS supplementation modulated serum bacteriostatic activity against non-pathogenic Escherichia coli and altered skin mucus lectin-binding patterns. Validation of the enhanced immune competence was confirmed by the enhancement of fish survival rate after exposure to E. tarda. Overall, the results demonstrate that dietary XOS, particularly at 3% inclusion, is effective in enhancing innate immunity and disease tolerance in olive flounder.