Draft Genome Sequence of the Type Strain Aeromonas salmonicida subsp. salmonicida ATCC 33658

Pathogenic characterization of Aeromonas salmonicida subsp. masoucida turbot isolate from China

Aeromonas salmonicida is a gram-negative bacterium that is the causative agent of furunculosis. An A. salmonicida strain was isolated from diseased turbot (Scophthalmus maximus) with the sign of furunculosis from North China. Based on vapA gene, the strain was further classified as A. salmonicida subsp. masoucida RZ6S-1. Culturing RZ6S-1 strain at high temperature (28°C) obtained the virulence attenuated strain RZ6S. Genome sequence comparison between the two strains revealed the loss of the type IV secretion system (T4SS) and type III secretion system (T3SS) from the native plasmid pAsmB-1 and pAsmC-1 of wild-type strain RZ6S-1, respectively. Further study demonstrated that the wild-type strain RZ6S-1, but not its derivative mutant RZ6S, can stimulate apoptosis. Elevated protein level of cleaved caspase-3 was detected from epithelioma papulosum cyprinid (EPC) cells infected with wild-type strain RZ6S-1 as compared with that infected with RZ6S strain. Meanwhile, the invasion of the mutant strain RZ6S was about 17-fold higher than the wild-type strain RZ6S-1, suggesting that some protein(s) from A. salmonicida subsp. masoucida RZ6S-1 suppress its invasion. The RZ6S mutant strain was attenuated, since its LD₅₀ is over 10,000 times higher compared to the wild-type strain as revealed in the turbot infection model.

Aeromonas salmonicida infection kinetics and protective immune response to vaccination in sablefish (Anoplopoma fimbria)

Effective vaccine programs against Aeromonas salmonicida have been identified as a high priority area for the sablefish (Anoplopoma fimbria) aquaculture. In this study, we established an A. salmonicida infection model in sablefish to evaluate the efficacy of commercial vaccines and an autogenous vaccine preparation. Groups of 40 fish were intraperitoneally (ip) injected with different doses of A. salmonicida J410 isolated from infected sablefish to calculate the median lethal dose (LD₅₀). Samples of blood, head kidney, spleen, brain, and liver were also collected at different time points to determine the infection kinetics. The LD₅₀ was estimated as ~3 × 10⁵ CFU/dose. To evaluate the immune protection provided by an autogenous vaccine and two commercial vaccines in a common garden experimental design, 140 fish were PIT-tagged, vaccinated and distributed equally into 4 tanks (35 fish for each group, including a control group). Blood samples were taken every 2 weeks to evaluate IgM titers. At 10 weeks post-immunization, all groups were ip challenged with 100 times the calculated LD₅₀ for A. salmonicida J410. A. salmonicida was detected after 5 days post-infection (dpi) in all collected tissues. At 30 days post-challenge the relative percentage survival (RPS) with respect to the control group was calculated for each vaccine. The RPS for the bacterin mix was 65.22%, for Forte Micro 4® vaccine was 56.52% and for Alpha Ject Micro 4® was 30.43%, and these RPS values were reflected by A. salmonicida tissue colonization levels at 10 days post-challenge. Total IgM titers peaked at 6-8 weeks post-immunization, where the autogenous vaccine group showed the highest IgM titers and these values were consistent with the RPS data. Also, we determined that the A. salmonicida A-layer binds to immunoglobulins F(ab)' in a non-specific fashion, interfering with immune assays and potentially vaccine efficacy. Our results indicate that vaccine design influences sablefish immunity and provide a guide for future sablefish vaccine programs.