Effects of Probiotics and Its Extracellular Products on the Growth Performance, Immune Response, and Aeromonas hydrophila Resistance of Grass Carp

The goal of this study was to research the effects of Lactobacillus buchneri L3-9 and its extracellular products on growth, immunity, intestinal microorganisms, and disease resistance to Aeromonas hydrophila in grass carp. A total of 120 fish (80–90 g) were distributed into 12 aquariums (10 fish/aquarium) in three replicates and fed 1% of their body weight for 21 days. The grass carp were fed four different diets: a control (group C), a diet supplemented with 1 × 10⁶ cell g⁻¹ of L. buchneri L3-9 (group B), a diet supplemented with extracellular products of L. buchneri L3-9 (group E), and a diet supplemented with L. buchneri L3-9 and the extracellular products (group BE). Compared with the control diet, the B, E, and BE diets increased the weight gain rate by 4.49%, 2.59%, and 4.38%, respectively (p  < 0.05), and significantly decreased the feed conversion ratio by 0.72, 0.57, and 0.76, respectively (p < 0.05). Groups B, BE, and E showed significant decreases in mortality, with values of 26.67%, 16.67%, and 23.33%, respectively (p < 0.05). Group B showed a significant increase in superoxide dismutase (SOD) activity and total antioxidant capacity (T-AOC) and decrease in malondialdehyde (MDA) content compared to the control group (p < 0.05). The highest catalase (CAT) activity was found in group E (p < 0.05). In group B, the expression of the gut proinflammatory gene TNF-α was downregulated (p < 0.05), and the expression of anti-inflammatory genes, including TGF-β1, IL-10, and Tlr-8, was upregulated compared to that in the control group (p < 0.05). Groups BE and E showed an increased abundance of Lactobacillaceae. Additionally, a drastic decrease in the abundance of pathogenic bacteria such as Aeromonadaceae and Enterobacteriaceae was in these groups compared to the control group. In conclusion, L. buchneri L3-9 and its extracellular products could improve the growth performance, immune responses, and resistance to A. hydrophila in grass carp. This study provides insights for the development and application of microecological preparations.