Dietary Scenedesmus ovalternus improves disease resistance of overwintering gibel carp (Carassius gibelio) by alleviating toll-like receptor signaling activation

Abstract

This study was conducted to investigate the effect of dietary Scenedesmus ovalternus on the growth and disease resistance of gibel carp (Carassius gibelio) during overwintering. Gibel carp (initial body weight: 90.39 ± 0.33 g) were fed with diets containing 0% or 4% Scenedesmus ovalternus (DS0 and DS4) for 4 weeks during the early overwintering period, and then all fish were left unfed during the late overwintering period. A bacterial challenge test using Aeromonas hydrophila was subsequently conducted. The 4% Scenedesmus ovalternus diet had no effect on the growth of gibel carp (P > 0.05), but did improve the survival rate after the challenge (P ≤ 0.05). In the DS0 group, the bacterial challenge decreased the contents of complement 3 (C3), immunoglobulin M (IgM), interleukin 2 (IL2) and tumor necrosis factor α (TNFα) in fish (P < 0.05); in the DS4 group, the challenge increased total antioxidant capacity (T-AOC) and myeloperoxidase (MPO) activity but decreased IL2 and TNFα contents (P < 0.05). The activities of MPO and contents of C3, IgM and TNFα were higher in the DS4 group than that fed the DS0 diet after bacterial challenge (P < 0.05). Compared to pre challenge, the expression levels of toll like receptor 2 (TLR2), toll like receptor 3 (TLR3), toll like receptor 4 (TLR4), myeloid differentiation factor 88 (MyD88), Toll/IL-1 receptor domain-containing adaptor protein (TIRAP), TIR-domain-containing adapter-inducing interferon β (TRIF), nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor α (IκBα), transforming growth factor β (TGFβ), interleukin 1β (IL1β), tumor necrosis factor α1 (TNFα1) and interleukin 10 (IL10) in the head kidney of gibel carp were induced after challenge (P < 0.05). Gibel carp fed the DS4 diet showed lower expression of TGFβ in head kidney before the challenge and lower expression of TLR2, TLR3, TLR4, TIRAP, TRIF, IκBα, TNFα1, IL10 and TGFβ after the challenge than that fed the DS0 diet (P < 0.05). Overall, Scenedesmus ovalternus supplement enhanced the resistances of gibel carp against A. hydrophila after overwintering via the TLR signaling pathway.