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Bacterium usually utilises type III secretion systems (T3SS) to deliver effectors directly into host cells with the aids of chaperones. Hence, it is very important to identify bacterial T3SS effectors and chaperones for better understanding of host-pathogen interactions. Edwardsiella piscicida is an invasive enteric bacterium, which infects a wide range of hosts from fish to human. Given E. piscicida encodes a functional T3SS to promote infection, very few T3SS effectors and chaperones have been identified in this bacterium so far. Here, we reported that EseK is a new T3SS effector protein translocated by E. piscicida. Bioinformatic analysis indicated that escH and escS encode two putative class I T3SS chaperones. Further investigation indicated that EscH and EscS can enhance the secretion and translocation of EseK. EscH directly binds EseK through undetermined binding domains, whereas EscS binds EseK via its N-terminal α-helix. We also found that EseK has an N-terminal chaperone-binding domain, which binds EscH and EscS to form a ternary complex. Zebrafish infection experiments showed that EseK and its chaperones EscH and EscS are necessary for bacterial colonisation in zebrafish. This work identified a new T3SS effector, EseK, and its two T3SS chaperones, EscH and EscS, in E. piscicida, which enriches our knowledge of bacterial T3SS effector-chaperone interaction and contributes to our understanding of bacterial pathogenesis. [ABSTRACT FROM AUTHOR]

Edwardsiella piscicida, a bacterial pathogen in fish and humans, expresses a type III secretion system (T3SS) that is critical for pathogen virulence and disease development. However, little is known about the associated effectors and their functional importance. In this study, we identified the ETAE_1757 encoded protein, termed here E. piscicida secretion effector H (EseH) as a novel T3SS effector. We found that upon infection with E. piscicida, EseH is translocated into nucleus of host cells which required the T3SS. Homology modelling analysis suggests that EseH is an enzyme that belongs to the family of phosphothreothine lyases. Consistently, EseH inhibited phosphorylation of ERK1/2, p38α and JNK MAPK pathways in host cells, but had no effect on the NF-kB pathway. Furthermore, mutation of the critical amino acid residues predicted to confer phosphothreonine lyase activity abolished the ability of EseH to inhibit phosphorylation of ERK1/2, p38α and JNK MAPK pathways in host cells. In addition, we found an increase in transcript levels of TNF-α, IL-12, IL-10 and IFN-γ in zebrafish infected with the eseH mutant when compared with the wild type bacterium. Importantly, the virulence of E. piscicida deficient in EseH was highly attenuated in the zebrafish infection model which correlated with decreased loads of the mutant bacterium in both liver and kidney. Complementation of the E. piscicida mutant strain with EseH restored virulence in zebrafish. These results identified EseH as a critical T3SS effector that contributes to virulence by targeting MAPK signalling during E. piscicida infection. [ABSTRACT FROM AUTHOR]