The Vaccine Candidates Against Major Fish Pathogens (Pseudomonas fluorescens and Aeromonas hydrophila) from the Outer Membrane Proteins of P. fluorescens in Fish

Vaccines have demonstrated greater efficiency in providing immunoprotection against bacterial species, making them potentially valuable in aquaculture. In this study, twenty-four outer membrane proteins (OMPs) of Pseudomonas fluorescens were cloned, purified, and 16 OMP mouse antisera were prepared, with titers all exceeding 1:3200. Subsequently, these 16 antisera were used to passively immunize crucian carp (Carassius auratus) followed by challenge with the pathogenic bacteria. The results showed that five OMP antisera (PF0542, SurA, PF1798, PF2253, and PF4616), as well as the whole OMP serum, provided immune protection rates exceeding 60% against P. fluorescens and Aeromonas hydrophila infection (p < 0.05). Moreover, these five OMP antisera reduced the mRNA expression of inflammatory cytokines and antioxidant factors (p < 0.05), and exerted protective effects on the structural integrity of the kidney, spleen, and intestinal tissues. In addition, active immunization of crucian carp with these five identified OMPs followed by pathogen challenge demonstrated that these proteins could activate non-specific immune responses in fish, confer immune protection against P. fluorescens and A. hydrophila infection, reduce the mRNA expression of inflammatory cytokines and antioxidant factors (p < 0.05), and protect the tissue structure of the kidney, spleen, and intestine. Collectively, these five OMPs (PF0542, SurA, PF1798, PF2253, and PF4616) can activate immune responses in crucian carp, confer protection against bacterial infection, and exhibit reductions in inflammatory/oxidative responses associated with protection following bacterial challenge, and as well as viscera structure-maintaining effects. Therefore, the five OMPs hold promise as vaccine candidates against bacterial infections (P. fluorescens and A. hydrophila) for both passive and active immunization in fish.