Characterization of a novel live attenuated Edwardsiella piscicida vaccine based on the overexpressed type III secretion system and systematic deletion of the associated effectors

Fur-mediated regulation of hydrogen sulfide synthesis, stress response, and virulence in Edwardsiella piscicida

The production of endogenous hydrogen sulfide (H2S) is an important phenotype of bacteria. H2S plays an important role in bacterial resistance to ROS and antibiotics, which significantly contributes to bacterial pathogenicity. Edwardsiella piscicida, the Gram-negative pathogen causing fish edwardsiellosis, has been documented to produce hydrogen sulfide. In the study, we revealed that Ferric uptake regulator (Fur) controlled H2S synthesis by activating the expression of phsABC operon. Besides, Fur participated in the bacterial defense against ROS and cationic antimicrobial peptides and modulated T3SS expression. Furthermore, the disruption of fur exhibited a significant in vivo colonization defect. Collectively, our study demonstrated the regulation of Fur in H2S synthesis, stress response, and virulence, providing a new perspective for better understanding the pathogenesis of Edwardsiella.

The QseE-QseF two-component system: A key mediator of epinephrine-regulated virulence in the marine pathogen Edwardsiella piscicida

Edwardsiella piscicida is a widespread pathogen that infects various fish species and causes massive hemorrhagic septicemia, resulting in significant property damage to the global aquaculture industry. Type III and VI secretion systems (T3/T6SS), controlled by the master regulator EsrB, are important virulence factors of E. piscicida that enable bacterial colonization and evasion from host immune clearance. In this study, we demonstrate that the QseE-QseF two-component system negatively regulated esrB expression by reanalysis of Tn-seq data. Moreover, the response regulator QseF directly bound to esrB promoter and inhibited the expression of T3/T6SS genes, especially in the presence of epinephrine. Furthermore, in response to the prompt increasing of epinephrine level, the host immune genes were delayed repressed and QseE-QseF timely inhibited the expression of T3/T6SS genes to evade immune clearance. In summary, this study enhances our understanding and knowledge of the conditional pathogenesis mechanism and virulence regulation network of E. piscicida.

CRISPR/Cas9-mediated generation of auxotrophic Edwardsiella piscicida mutants and immunization in olive flounder (Paralichthys olivaceus)

Edwardsiella piscicida has been a cause of mass mortality in cultured fish. In this study, to produce auxotrophic E. piscicida mutants, a CRISPR/Cas9 system was used instead of the traditional sacB-based allelic exchange method. Under the optimal CRISPR engineering condition, we could efficiently produce either alr or asd gene knockout E. piscicida auxotrophic mutants, and this genome editing process was much simpler and faster than the allelic exchange method. The simultaneous knockout of double auxotrophic genes (alr and asd) and the insertion of a foreign gene expression cassette in E. piscicida chromosome were also successfully performed using the established CRISPR/Cas9 system. Furthermore, to enhance the possibility to get permission as a commercial vaccine, we produced an auxotrophic E. piscicida mutant having only one nucleotide-deleted alr gene (E. piscicida △alr-1). Olive flounder (Paralichthys olivaceus) fingerlings immunized with 1 × 10⁶ and 1 × 10⁵ CFU/fish of E. piscicida △alr-1 showed the superior ability in the induction of serum agglutination activity and in the protection against E. piscicida compared to killed E. piscicida. However, olive flounder immunized with 1 × 10⁷ CFU/fish of E. piscicida △alr-1 showed high mortality far before the challenge, and the isolated E. piscicida from moribund and dead fish had the wild type alr gene, suggesting the reversion of one base-deleted alr gene to original form by a second mutation in olive flounder. Therefore, investigation on the minimum number of edited nucleotide for stable maintenance of E. piscicida mutants should be further conducted.

Interplay between ferric uptake regulator Fur and horizontally acquired virulence regulator EsrB coordinates virulence gene expression in Edwardsiella piscicida

Edwardsiella piscicida mediates hemorrhagic septicemia and is a leading pathogen of fish. E. piscicida invades and colonizes macrophages using type III and VI secretion systems (T3/T6SS) that are controlled by a two-component system (TCS) EsrA-EsrB. Iron acquisition is essential for E. piscicida pathogenesis and coordination between iron and TCS signaling in modulating bacterial virulence is not well understood. Here, we show that iron uptake systems are co-regulated by ferric uptake regulator (Fur) in E. piscicida. Fur bound to 98 genes that harbored conserved Fur-box to globally control the expression of ∼755 genes, including those encoding iron uptake systems, T3/T6SS, and Icc, cAMP phosphodiesterase that represses biofilm formation. Additionally, Fur, in complex with iron, bound to the esrB promoter to repress expression and ultimately attenuated virulence. Conversely, EsrB activated the expression of T3/T6SS and iron uptake systems to mitigate a shortage of intracellular iron during iron scarcity. Furthermore, EsrB directly bound to and activated the fur promoter, leading to Fur-ferrous ion-dependent esrB repression in the presence of iron. Finally, Fur-EsrB interplay was essential for bacterial fitness during in vivo infection and survival in seawater environments. Collectively, we highlight the mechanisms that underlie the reciprocal regulatory networks of iron homeostasis and virulence systems in E. piscicida.