An isocitrate lyase gene-deleted strain ofin live attenuated vaccine development against fish nocardiosis.

Abstract

BACKGROUND: Fish nocardiosis caused byis a severe bacterial disease in aquaculture, causing significant economic losses. The effectiveness of antibiotics in dealing with nocardiosis induced byis not satisfactory. NsICL is a potential virulence factor inbased on its crucial role in the glyoxylate cycle, its identity as a secreted protein, and its established role in virulence and intracellular survival in homologous pathogens such as. so, the NS-ΔICL deletion strain was constructed to investigate its function. METHODS: Thegene was knocked out via homologous recombination to generate the mutant strain NS-ΔICL. Comparative analysis of morphology, growth, and virulence was performed between the mutant and wild-type strains. The NS-ΔICL strain was used to immunize hybrid snakehead (♀ ×♂), with immune responses evaluated through non-specific immune parameters, specific antibodies, and expression levels of immune-related genes. Protective efficacy was assessed by challenge tests. RESULTS: NS-ΔICL showed reduced virulence (LD= 2.60 × 10cfu/fish) compared to the wild-type strain (LD= 4.32 × 10cfu/fish). Immunized fish exhibited significantly higher levels of non-specific immune factors (AKP, ACP, POD, LZM) and specific IgM antibodies. Immune-related genes (α,) were upregulated in vaccinated fish. Relative percentage survival (RPS) was 77.03% in vaccinated fish after challenge tests, which indicating strong protective efficacy. CONCLUSION: The NS-ΔICL deletion strain was successfully constructed in this study. It can not only induce humoral and cellular immunity in fish but also effectively protect fish againstinfection. These results provide a foundation for the development of a live attenuated vaccine for treating fish nocardiosis.