Development and preliminary evaluation of a ΔtssB-EP as a live attenuated vaccine against Edwardsiella piscicida.

BACKGROUND: Vaccination is the most effective strategy for preventing and controlling infectious diseases in aquaculture. Edwardsiella piscicida, a Gram-negative facultative intercellular bacterial pathogen, is the etiological agent of edwardsiellosis in fish, a disease associated with significant morbidity, mortality, and economic losses in the aquaculture industry. METHODS: In this study, a live attenuated mutant strain, designated ΔtssB-EP, was generated from a virulent Edwardsiella piscicida by in-frame deletion of the tssB gene, which encodes a component of the Type VI secretory system, using an allelic exchange approach. A series of in vitro and in vivo studies was conducted to evaluate the potential of the developed mutant strain as an effective vaccine candidate. RESULTS: The precise deletion of about 506 bp was confirmed by PCR. The studies demonstrated that the ΔtssB-EP (mutant) exhibited a significant reduction in biofilm formation, along with a 2.7 log10 increase in lethal dose (LD50) compared to the wild-type strain. In addition, a vaccination trial was conducted in which Labio rohita juveniles were immunized via intraperitoneal (I.P.) injection with a sublethal dose of the ΔtssB-EP mutant. At 35 days post-vaccination (dpv), the fish were challenged with wild-type E. piscicida to evaluate protection against mortality. After vaccination, antibody levels were significantly higher in the vaccinated group than in the control group (PBS/Mock-vaccinated). The improved relative percentage survival (RPS = 70.83%) was observed in the group of L. rohita juveniles administered with the ΔtssB-EP mutant. CONCLUSION: The results confirm significant attenuation of virulence in the ΔtssB-EP mutant, which elicited a robust immune response and conferred substantial protection against mortality from E. piscicida infection in Labeo rohita.