Efficacy of formalin-killed Pseudomonas anguilliseptica vaccine on immune gene expression and protection in farmed olive flounder, Paralichthys olivaceus

Formalin killed Pseudomonas anguilliseptica bacterial vaccine was prepared and administered to farm reared olive flounder, Paralichthys olivaceus reared at 17 °C and 20 °C for 4 weeks. Non-vaccinated fishes (n = 150) served as positive control. Vaccinated fishes were divided into two groups (n = 150 each in replicate). Both the vaccinated and non-vaccinated fishes were challenged intraperitoneally with P. anguilliseptica (1 × 107 CFU ml−1) isolates and PBS (negative control). Fishes were sampled from zero hour post injection (hpi) for 28 days (each hour and each day); the mean percent mortality and relative percent survival (RPS) were calculated for the challenged and control groups. The vaccinated fishes had a significant increase in RPS (69 and 89, respectively); the percentage mortality declined from 83 ± 0.6 and 74 ± 0.7 in challenged and control fishes to 25% ± 0.8% and 8% ± 0.8% in vaccinated and challenged fish groups, respectively. The immune gene expression assay was analyzed using real-time PCR. Vaccinated fishes registered a significant increase in the expression of TNFR-1, FasL, IRF7, TLR2, IL-1b and CD40 gene transcripts when compared to the control group. The upregulation of these genes along with the increased RPS values suggest that the formalin-killed cells of P. anguilliseptica could play an important role in immunizing olive flounder against P. anguilliseptica.