Dietary Bacillus velezensis R-71003 and sodium gluconate improve antioxidant capacity, immune response and resistance against Aeromonas hydrophila in common carp

This study aimed to evaluate the effects of dietary supplementation with Bacillus velezensis R-71003 combined with sodium gluconate on antioxidant capacity, immune response and resistance against Aeromonas hydrophila in common carp . In addition, the biocontrol potential of the secondary metabolites of B. velezensis R-71003 was also evaluated to analyze the possible mechanism of B. velezensis R-71003 against A. hydrophila . The results indicated that the antibacterial crude extract of B. velezensis R-71003 can destroy the cell wall of A. hydrophila . Moreover, the results showed that dietary B. velezensis R-71003 could promote antioxidant capacity, which significantly increased the activities of CAT and SOD and decreased the content of MDA. Additionally, B. velezensis R-71003 supplementation significantly enhanced the immunity of common carp, as measured by the mRNA expression levels of cytokine-related genes ( TNF-α , TGF-β , IL-1β and IL-10 ). In addition, dietary B. velezensis R-71003 exhibited an upregulation of IL-10 and a downregulation of IL-1β, coupled with higher survival rates when challenged with A. hydrophila compared to the positive group. Furthermore, compared to prechallenge, the mRNA expression levels of TLR-4 , MyD88 , IRAK1 , TRAF6 , TRIF and NF-κB in the head kidney of common carp were significantly increased after challenge. The fish fed the B. velezensis R-71003 diet showed lower expression of TLR-4 , MyD88 , IRAK1 , TRAF6 , TRIF and NF-κB after the challenge than those fed the control diet. Thus, this study revealed that B. velezensis R-71003 can improve the resistance of common carp to pathogenic bacteria by destroying bacterial cell walls and improving fish immunity by activating the TLR4 signaling pathway . Importantly, this study indicated that sodium gluconate has a positive effect on B. velezensis R-71003 in enhancing the anti-infection ability of common carp. The results of this study will lay the foundation for the application of B. velezensis R-71003 in combination with sodium gluconate as an alternative to antibiotics in aquaculture.