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 log 10 increase in lethal dose (LD 50 ) 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.