Optimal dietary glycerol monolaurate supplementation improves growth performance, antioxidant capacity, immune responses and intestinal microbiota in juvenile largemouth bass

Introduction Plant-derived functional additives have gained increasing attention as sustainable alternatives in aquafeed. This study aims to investigate the effects of dietary monolaurin on productive performance, innate immune indicators, antioxidative effects, and gut flora in largemouth bass ( Micropterus salmoides ). Methods Fish were fed diets containing 0, 500, 800, 1100, and 1400 mg/kg GML for 56 days, with three replicates per treatment. Results The results showed that GML supplementation significantly improved growth performance. Final body weight was higher in the 800 and 1100 mg/kg groups, while weight gain rate and specific growth rate peaked at 800 mg/kg. Whole-body crude lipid content decreased in the 500 and 800 mg/kg groups, whereas crude protein content increased across all GML-supplemented groups. GML also enhanced antioxidant capacity by increasing total antioxidant capacity and catalase and superoxide dismutase activities while reducing malondialdehyde levels. Serum acid phosphatase and alkaline phosphatase activities peaked in the 800 and 1100 mg/kg groups. The expression levels of il6 and tnfa were significantly downregulated in the 800 and 1100 mg/kg groups, while those of tgfb1a and il22 were significantly upregulated. Intestinal microbiota analysis revealed that GML reshaped the gut microbial community. The relative abundances of Aeromonas and Pseudomonas decreased at 800–1400 mg/kg, whereas those of Cetobacterium and Bacteroides increased at 800 mg/kg and 1100 mg/kg. Regression analysis based on final body weight estimated the optimal dietary GML level at 750 mg/kg. Discussion These findings indicate that appropriate dietary GML supplementation can promote growth while improving antioxidant status, selected innate immune responses, and intestinal microbial balance in largemouth bass. Thus, GML may serve as a promising plant-derived functional feed additive in largemouth bass aquaculture.