A bivalent oral yeast vaccine displaying Nocardia seriolae FHA and LMBV MCP confers protection in largemouth bass (Micropterus salmoides).

Largemouth bass ranavirus (LMBV) and Nocardia seriolae are major pathogens affecting largemouth bass (Micropterus salmoides), causing significant economic losses. In this study, recombinant yeasts expressing the fibronectin-binding protein A (FHA) of N. seriolae or the major capsid protein (MCP) of LMBV were constructed using the yeast surface display (YSD) system. Based on these recombinant strains, a bivalent oral yeast-based vaccine was developed and its protective efficacy was systematically evaluated. Oral administration of the vaccine induced specific antibodies against FHA and MCP in serum, as well as increased the transcription levels of adaptive immune genes (IgM, IgT, MHC-II) and innate immune genes (IL-1β, TNF-α, IFN-1, Mx2) in the hindgut, liver, and spleen. Importantly, the bivalent oral vaccine provided significant protection against both pathogens in largemouth bass, with relative percent survival (RPS) values of 56.7% against N. seriolae and 63.3% against LMBV. Meanwhile, the oral vaccine reduced pathogen loads and alleviated histopathological lesions. In summary, these findings suggest that the bivalent oral vaccine developed in this study could serve as a promising strategy for controlling N. seriolae and LMBV infections in largemouth bass aquaculture.