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

Abstract

<i>Aeromonas veronii</i> is a pathogen capable of infecting humans, livestock and aquatic animals, resulting in serious economic losses. In this study, two recombinant <i>Lactobacillus casei</i> expressing flagellin A (FlaA) of <i>A. veronii</i>, Lc-pPG-1-FlaA (surface-displayed) and Lc-pPG-2-FlaA (secretory) were constructed. The immune responses in fish administered with recombinant <i>L. casei</i> were evaluated. The two recombinant <i>L. casei</i> 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 <i>L. casei</i> was significantly (<i>p</i> < 0.05) upregulated, which indicated that recombinant <i>L. casei</i> could activate the innate immune system to trigger the cell immune response and inflammatory response. Furthermore, recombinant <i>L. casei</i> was able to survive the intestinal environment and colonize in intestine mucosal. The study showed that after being challenged by <i>A. veronii</i>, 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 <i>L. casei</i> might prevent <i>A. veronii</i> infection by activating the immune system to trigger immune responses. We demonstrated that flagellin as an antigen of vaccine, is acceptable for preventing <i>A. veronii</i> infection in fish. The recombinant <i>L. casei</i> expressing FlaA may be a novel mucosal vaccine for treating and controlling <i>A. veronii</i>.