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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. [ABSTRACT FROM AUTHOR]

The type III secretion system effector EseJ plays a regulatory role inside bacteria. It suppresses the adherence of Edwardsiella piscicida (E. piscicida) to host epithelial cells by down regulating type 1 fimbriae. In this study, we observed that more macrophages infected with ΔeseJ strain of E. piscicida detached as compared with those infected with the wild‐type (WT) strain. Terminal deoxynucleotidyl transferase dUTP nick‐end labelling (TUNEL) staining and cleaved caspase‐3 examination revealed that the detachment is due to increased apoptosis, suggesting that EseJ suppresses macrophage apoptosis. However, apoptosis inhibition by EseJ is not relative to a type III secretion system (T3SS) and is not related to EseJ's translocation. Since EseJ negatively regulates type 1 fimbriae, murine J774A.1 cells were infected with ΔeseJΔfimA or ΔeseJΔfimH strains. It was demonstrated that ΔeseJ stimulates macrophage apoptosis through type 1 fimbriae. Moreover, we found that infecting J774A.1 cells with the ΔeseJ strain increased levels of cleaved caspase‐8, caspase‐9, and caspase‐3, demonstrating that EseJ inhibits apoptosis through either an extrinsic or a combination of extrinsic and intrinsic pathways. Pre‐treatment of macrophages with caspase‐8 inhibitor prior to infection with the ΔeseJ strain decreased the levels of cleaved caspase‐8, caspase‐9, and caspase‐3, indicating that the ΔeseJ strain stimulates apoptosis, mainly through an extrinsic pathway by up regulating type 1 fimbriae. Zebrafish larvae or blue gourami fish infected with the ΔeseJ strain consistently exhibited higher apoptosis than those infected with the E. piscicida WT strain or ΔeseJΔfimA strain. Taken together, we revealed that the T3SS protein EseJ of E. piscicida inhibits host apoptosis, mainly through an extrinsic pathway by down regulating type 1 fimbriae. [ABSTRACT FROM AUTHOR]