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

## TL;DR

Adding xylooligosaccharide to the diet of olive flounder improves their growth, immune system, and ability to survive bacterial infections.

## Contribution

This study demonstrates that 3% dietary xylooligosaccharide enhances innate immunity and disease resistance in olive flounder.

## Key findings

- XOS improved growth performance and liver and visceral health in olive flounder.
- XOS enhanced innate immune parameters like lysozyme, antiprotease, and complement activities.
- Fish with higher XOS levels showed increased survival after bacterial infection.

## Abstract

Olive flounder (Paralichthys olivaceus) is an important flatfish species in marine aquaculture, where improving disease resistance through nutrition is a practical strategy to enhance production sustainability. This study evaluated the effects of dietary supplementation with xylooligosaccharide (XOS) (a prebiotic) at levels of 0–3% over an eight-week feeding period. Increasing dietary XOS improved growth performance and positively influenced several indicators of liver and visceral health. XOS supplementation also enhanced key components of innate immunity, including serum lysozyme, antiprotease, and complement activities, and skin mucus immune parameters. Importantly, fish fed higher XOS levels showed increased survival following a bacterial challenge with Edwardsiella tarda, indicating improved disease resistance. Based on the combined growth, immune, and survival responses, a dietary inclusion level of 3% XOS is recommended to strengthen innate immunity and enhance tolerance to bacterial infection in olive flounder culture.

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.