Vaccination efficiency of surface antigens and killed whole cell of Pseudomonas putida in large yellow croaker (Pseudosciaena crocea).

Large yellow croaker (Pseudosciaena crocea), a major marine fish aquacultured in the southeastern coastal region of China, has become endangered by the pathogen Pseudomonas putida in recent years. P. putida infections occur in low water temperatures when fish reduce food intake, thus oral antibiotic administration is not practical. Therefore, vaccination may be the only method to prevent the infection. In the present study, main surface antigens of P. putida, including lipopolysaccharide (LPS), outer membrane proteins (OMP), extracellular biofilm polysaccharide (EPS), and formalin-killed cell (FKC) bacterin, were prepared and the fish vaccinated. On post-immunization day 28, serum antibody titers, phagocytic responses of leukocytes, and lysozyme activities of the fish were evaluated. The efficiency of vaccination was tested by artificial challenge via intraperitoneal injection of live bacteria on post-immunization day 28 and 35, respectively. The results showed that although significant humoral and innate immune responses were elicited in all vaccination groups, the challenge produced similar poor protection in both tests, with a relative percent survival (RPS) of 0-40%. Although the EPS group showed a complete lack of protection, LPS reached the highest RPS value (40%), suggesting that LPS may be involved in protection immunity against the pathogen. Further analysis of the ultra-structures of tissues from infected fish via TEM revealed macrophage survival and intracellular replication ability of the pathogen. New strategies for development might put more emphasis on efficient clearance of intracellular bacteria. The present study is the first to report vaccination against the fish pathogen P. putida and the first investigation of intracellular survival of this pathogen in host macrophages.