Pathogenicity of Edwardsiella anguillarum to American Eels(Anguilla rostrota)and the Expression of Host Anti-E.anguillarum Infection Related Genes
In this study,American eels were infected with 1.0 × 107-2.0 × 108 cfu/mL of E.anguil-larum to determine the LD50 of E.anguillarum to American eels.Additionally,American eels with an average body weight of 20 g were injected with 3.3 × 106 cfu/fish of E.anguillarum to assess the bacterial load in the liver,kidney and blood at 6 time points of 12,24,36,48,60 and 72 hours post infection.Furthermore,the pathological changes of liver,kidney and spleen of eels were observed 36 h and 60 h after the infection.The mRNA expression of the outer membrane protein A(OmpA)of the pathogen and 16 anti-infection relat-ed proteins of the host was also detected at 4 time points:before infection,and at 12,36 and 60 h post-infec-tion,using the qRT-PCR method.The results showed that the LD50 of E.anguillarum to American eels was 2.5 × 105 cfu/g body weight(equivalent to 5.0 × 106 cfu/fish).The plate colony count results demonstra-ted that the bacterial load in the kidney of American eel was significantly higher(P＜0.05)than that in the liver and blood at 36,48 and 60 h post-infection.The bacterial load in the kidney and blood peaked at 36 h,while in the liver,it peaked at 72 h post-infection.Pathological observations revealed congestion in liver ves-sels and atrophy of glomeruli at 36 hours post-infection.Exfoliation of the epithelial cells in renal tubules and thrombosis formation in spleen vessels were also observed.At 60 hours post-infection,hepatic blood vessels showed thrombosis and necrosis of hepatocytes,while the epithelial cells of renal tubules became vacuolar,degenerated,and fell into the lumen of the tubules.Moreover,a significant number of necrotic nodules caused by bacterial infection were seen in the spleen.Comparison of gene expression levels at different time points revealed that the expression of the OmpA gene in the liver increased significantly(P＜0.05)at 12 hours post-infection.In the spleen,OmpA expression decreased initially and then sharply increased,while in the kidney,it displayed an initial increase,followed by a decrease,and eventually another increase.The expres-sion of 16 immune-related genes in response to E.anguillarum infection demonstrated that the expression lev-els of 5 immunoglobulin-related genes in the spleen were significantly higher(P＜0.05)at the early stage of infection compared to the later stage.However,no significant differences were found in the liver and kidney at different time points.Additionally,the expression of 3 tumor necrosis factor-related genes was relatively high at the early stage of infection.Some immune-related genes also exhibited higher expression levels at 12 hours post-infection compared to other time points.
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