Dietary Bacillus velezensis FL9 enhances growth, intestinal health and Aeromonas hydrophila resistance in Cyprinus carpio.

This study aimed to evaluate the effects of dietary supplementation with Bacillus velezensis FL9 on growth performance, immune response, intestinal health, and disease resistance of Cyprinus carpio. A total of 420 fish (initial weight of 1.86 ± 0.25 g) were randomly assigned into four groups: a control group (C) and three treatment groups fed diets supplemented with B. velezensis FL9 at 1 × 105 CFU/g (SC), 1 × 107 CFU/g (MC), and 1 × 109 CFU/g (LC), respectively. After 56 days of feeding, ten fish per group were intraperitoneally challenged with Aeromonas hydrophila (2 × 107 CFU/mL) for 48 h. The results showed that dietary B. velezensis FL9, especially at 1 × 109 CFU/g, significantly improved growth performance, evidenced by increased weight gain rate (WGR) and a reduced feed conversion rate (FCR) (P < 0.05). The LC group enhanced liver antioxidant and immune-related enzyme activities (ACP, AKP, SOD, GST), reduced MDA content, improved intestinal digestive enzyme activities (lipase, pepsin, and α-amylase), and upregulated liver expression of immune-related genes (C3, LYZ, TLR1, TLR2) (P < 0.05). Moreover, the experimental treatment significantly improved the intestinal morphology, including villus height and muscular thickness. It also improved intestinal morphology and increased microbial diversity, with Cetobacterium becoming dominant in the MC group. After A. hydrophila challenge, the experimental groups showed significantly lower mortality, elevated ACP activity, and a modulated liver expression of immune-related genes (C3, LYZ, TLR1, TLR2, IL-8) (P < 0.05). Microbiota analysis revealed that, alongside Aeromonas, the relative abundance of beneficial bacteria in the experimental groups. In summary, dietary B. velezensis FL9 can improve growth performance, immunity, intestinal health, and disease resistance in C. carpio. Furthermore, our study found that the optimal concentration of B. velezensis FL9 in the diet was 1 × 109 CFU/g.