Synergistic effects of dietary supplementation of Bacillus subtilis WB60 and mannanoligosaccharide (MOS) on growth performance, immunity and disease resistance in Japanese eel, Anguilla japonica

Abstract

This study evaluated the synergistic effects of dietary Bacillus subtilis WB60 and mannanoligosaccharide (MOS) in juvenile Japanese eel, Anguilla japonica. Seven treatment diets were formulated to contain three different levels of B. subtilis (0.0, 0.5, and 1.0 × 10⁷ CFU/g diet denoted as BS₀, BS_0.5, and BS₁, respectively) with two MOS levels (0 and 5 g/kg diet denoted as M₀ and M₅, respectively), and one diet with oxytetracycline (OTC) at 5 g/kg diet. Each diet (BS₀M₀ (CON), BS₀M₅, BS_0.5M_0, BS_0.5M_5, BS₁M_0, BS₁M₅, and OTC) was fed to triplicate groups of 20 fish averaging 9.00 ± 0.11 g (mean ± SD) for eight weeks. Average weight gain, feed efficiency, specific growth rate and protein efficiency ratio of fish fed the BS_0.5M₅ and BS₁M₅ diets were significantly higher than those of fish fed CON, BS_0.5M₀ and OTC diets (P < 0.05). Significant increases in the nonspecific enzymatic activities (e.g., lysozyme and myeloperoxidase) were detected from fish fed the BS_0.5M₅, BS₁M₅, and OTC diets compared to the CON, BS_0.5M₀, and BS₀M₅ diets (P < 0.05). Whereas, immunoglobulin M expressions were recorded significantly higher for fish fed the BS_0.5M₅ and BS₁M₅ diets compared to those of fish fed the other diets (P < 0.05). Also, heat shock protein 70 mRNA levels of fish fed BS_0.5M₅ and BS₁M₅ diets were significantly higher than those of fish fed the CON diet (P < 0.05). Histological observations of the intestinal morphology showed healthier gut for fish fed BS_0.5M₅ and BS₁M₅ diets than those fed CON, BS₀M₅, and OTC diets. Additionally, resistance to bacterial challenge with Vibrio anguillarum was recorded significantly lower for fish fed the CON diet than those fed other diets (P > 0.05). Therefore, the results for growth performance, non-specific immune responses, intestinal morphology, and disease resistance demonstrated that supplementation of B. subtilis at 0.5 × 10⁷ CFU/g diet and mannanoligosaccharide at 5 g/kg diet could have beneficial synergistic effects in Japanese eel. The isolated probiotic from eel and the selected prebiotic could lead to the development of a specific and potential synbiotic in Japanese eel aquaculture.

Keywords: Bacillus; Immunity; Japanese eel; Mannanoligosaccharide; Synbiotic.