Evaluation of MarR-deleted Streptococcus agalactiae as a live-attenuated vaccine candidate for Nile Tilapia (Oreochromis niloticus)

Abstract

Streptococcus agalactiae is the dominant etiological agent inducing streptococcosis, which is the major threat to global tilapia aquaculture. Live-attenuated vaccines can induce robust cellular and humoral immunity, and have been proven to provide effective protection against S. agalactiae. Selecting an appropriate target is crucial for the development of attenuated vaccine. In this study, we constructed a mutant strain (ΔmarR) lacking the gene encoding the multiple antibiotic resistance regulator (MarR) to evaluate its vaccine potential. Comparative survival assays revealed that the ΔmarR exhibited significantly reduced blood survival relative to the wild-type (WT) HN016 strain. Additionally, the ΔmarR showed decreased hemolytic activity, anti-phagocytosis ability, adhesion and invasion capabilities. More importantly, its pathogenicity in tilapia was markedly attenuated with an LD₅₀ value 356-fold higher than HN016. Furthermore, immunization by intracelomic injection with ΔmarR conferred significant protection in tilapia against subsequent challenge with HN016 at 30 days post-vaccination, with 95 % vaccinated tilapia surviving for longer than 30 days and relative percent survival of 93.99 %. To investigate the immune response, we quantified immunity-related genes expression in head kidney and spleen. The results demonstrated that vaccination with ΔmarR could induce a robust and sustained adaptive immune response in tilapia. Taken together, the findings indicate that the marR gene deletion causes significant attenuation of S. agalactiae and the ΔmarR mutant emerges as a promising vaccine candidate to combat S. agalactiae infection.