Oral Administration of Lactobacillus Casei Expressing Flagellin A Protein Confers Effective Protection against Aeromonas Veronii in Common Carp, Cyprinus Carpio

Aeromonas veronii is a pathogen capable of infecting humans, livestock and aquatic animals, resulting in serious economic losses. In this study, two recombinant Lactobacillus casei expressing flagellin A (FlaA) of A. veronii, Lc-pPG-1-FlaA (surface-displayed) and Lc-pPG-2-FlaA (secretory) were constructed. The immune responses in fish administered with recombinant L. casei were evaluated. The two recombinant L. casei were orally administered to common carp, which stimulated high serum IgM and induced higher ACP, AKP, SOD and LYZ activity. Using qRT-PCR, the expression of IL-10, IL-8, IL-1β, TNF-α and IFN-γ in the tissue of fish immunized with recombinant L. casei was significantly (p < 0.05) upregulated, which indicated that recombinant L. casei could activate the innate immune system to trigger the cell immune response and inflammatory response. Furthermore, recombinant L. casei was able to survive the intestinal environment and colonize in intestine mucosal. The study showed that after being challenged by A. veronii, fish administered with Lc-pPG-1-FlaA (70%) and Lc-pPG-2-FlaA (50%) had higher survival rates compared to Lc-pPG and PBS, indicating that recombinant L. casei might prevent A. veronii infection by activating the immune system to trigger immune responses. We demonstrated that flagellin as an antigen of vaccine, is acceptable for preventing A. veronii infection in fish. The recombinant L. casei expressing FlaA may be a novel mucosal vaccine for treating and controlling A. veronii.