The contributions of fliG gene to the pathogenicity of Pseudomonas plecoglossicida and pathogen-host interactions with Epinephelus coioides

Pseudomonas plecoglossicida is a Gram-negative aerobic rod-shaped bacterium with polar flagella. It is the causative agent of visceral white spot disease in cultured fish, resulting in serious economic losses. In our previous study, RNA sequencing showed that the expression of the fliG gene in P. plecoglossicida is significantly up-regulated during infection of orange-spotted grouper (Epinephelus coioides). In this study, four P. plecoglossicida RNA interference (RNAi) mutants were successfully constructed by linking four short hairpin RNAs (shRNAs), which target different sites of the fliG gene, to pCM130/tac, respectively. The mRNA expression levels of the fliG gene in P. plecoglossicida were significantly decreased in four mutants. The shRNA-335 mutant (fliG-RNAi strain) showed the best silencing efficiency (88.2%) and was thus chosen for further analysis. Electron microscopy indicated that the flagella of the fliG-RNAi strain of P. plecoglossicida were shorter and finer than those of the wild type strain. The fliG-RNAi strain also showed significantly decreased mobility, chemotaxis, adhesion, and biofilm formation. Furthermore, compared with wild type strain infection, E. coioides infected with the fliG-RNAi strain exhibited a 0.5-d delay in the time of first death and 55% reduction in accumulated mortality, as well as milder splenic symptoms. RNAi of the fliG gene significantly affected the transcriptomes of both pathogen and host in the infected spleens of E. coioides. KEGG analysis revealed that the flagellar assembly pathway, bacterial chemotaxis pathway, and starch and sucrose metabolism pathway were significantly enriched in the pathogen at 3 days post infection (dpi). In contrast, the complement and coagulation cascade pathway and antigen processing and presentation pathway were significantly enriched in the host at 3 dpi. More immune-related pathways were enriched at 5 dpi and more differentially expressed genes were found in the complement and coagulation cascade and antigen processing and presentation pathways. Cytokine-cytokine receptor interaction, hematopoietic cell lineage, and IgA-producing intestinal immune network pathways were significantly enriched in the host at 5 dpi. These results indicate that fliG is an important virulence gene of P. plecoglossicida and contributes to the pathogenicity of P. plecoglossicida as well as pathogen-host interactions with E. coioides.