Dietary L-Tryptophan Improves Growth, Antioxidant Status, Intestinal Barrier, and Resistance to Aeromonas hydrophila Infection in Largemouth Bass

Tryptophan (Trp) has been well documented to positively regulate nutrient metabolism, immune responses, and intestinal microecological homeostasis in mammals. However, relevant studies regarding its functions in largemouth bass remain limited. Herein, we assessed the impacts of dietary Trp on growth, antioxidant status, intestinal barrier, and disease resistance in largemouth bass. Fish were fed a basal control diet (C) or basal diets supplemented with 0.1%, 0.2%, 0.4%, and 0.8% L‑Trp for 60 days. Dietary L-Trp supplementation significantly improved FBW, WGR, and SGR of largemouth bass, with the maximum values found in the 0.4%Trp group. Based on the WGR and SGR data, quadratic regression analysis further identified 0.49% as the optimal dietary L-Trp inclusion level. In terms of antioxidant capacity, the 0.4%Trp group exhibited markedly higher serum and intestinal activities of CAT, GSH-PX, and SOD, as well as a higher content of T-AOC, compared to the C group. Moreover, the 0.4%Trp group significantly upregulated intestinal Nrf2 expression and downregulated Keap1 expression. Relative to the C group, dietary Trp supplementation significantly elevated intestinal villus height and muscular layer thickness in the 0.1%, 0.2%, and 0.4% Trp groups. Consistently, the mRNA levels of intestinal tight junction genes (ZO-1, Claudin-1, and Claudin-4) were upregulated in the 0.4%Trp group. Supplementation with 0.4%Trp significantly downregulated the expression of proinflammatory cytokines (TNF-α, IL-1β, and IL-8) and upregulated the expression of antiinflammatory cytokines (TGF-β1, IL-10, IL-22, and AHR). Intestinal microbial analysis further revealed that dietary L-Trp supplementation enriched the abundance of beneficial bacteria, reduced pathogenic microbial populations, and improved intestinal microbial diversity and richness. Finally, dietary L-Trp supplementation effectively improved the survival rate of largemouth bass following an Aeromonas hydrophila challenge. This was associated with enhanced serum activities of LZM, AKP, and ACP, as well as increased intestinal T-SOD activity. Additionally, L-Trp supplementation upregulated the expression of Nrf2, TGF-β1, IL-10, and IL-22. In contrast, the 0.4%Trp group reduced the MDA content and downregulated expression of Keap1, HSP70, TNF-α, IL-1β, and IL-8. Collectively, 0.4% dietary Trp strengthens antioxidant defense, remodels intestinal microbiota, improves intestinal physical and immune barriers, and enhances resistance against A. hydrophila infection in largemouth bass.