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Diseases caused by Vibrio harveyi lead to severe economic losses in the aquaculture industry. Adhesion is an important disease-causing factor observed in bacteria with chemotactic activity. In our study, we measured the adhesion of V. harveyi by subjecting the bacteria to stress using Cu2+, Pb2+, Hg2+, and Zn2+. The genes responsible for chemotaxis (cheA , cheB , cheR , cheV , and cheY), which are also crucial for adhesion, were identified and silenced via RNAi. We observed that a decrease in chemotactic gene expression reduced the ability of the organism to demonstrate adhesion, motility, chemotaxis, and biofilm formation. Upon comparing the cheA -RNAi bacteria to the wild-type strain, we observed that the transcriptome of V. harveyi was significantly altered. Additionally, the expression of key genes and the adhesion ability were affected by the pH (pH of 5, 6, 7, 8, and 9), salinity (NaCl at concentrations of 0.8, 1.5, 2.5, 3.5, or 4.5%), and temperature (4, 15, 28, 37, and 44°C) of the medium. Based on these results, the following conclusions were made: (1) The chemotactic genes cheA , cheB , cheR , cheV , and cheY may regulate the adhesion ability of V. harveyi by affecting bacterial motility, and participate in the regulation of adhesion at different temperatures, salinities, and pH values; (2) stable silencing of cheA could alter the transcriptional landscape of V. harveyi and regulate the expression of genes associated with its adhesion mechanisms. [ABSTRACT FROM AUTHOR]

• A significant decrease in gene expression of adhesion-deficient V. harveyi. • fliA , fliR , and flrB were responsible for flagellation and silenced via RNAi. • fliA , fliR , and flrB have important roles in the adhesion of V. harveyi. • fliA , fliR , flrB regulate bacterial adhesion by affecting its motility, biofilm formation. • fliA , fliR , flrB regulate adhesion ability of V. harveyi in different environments. Bacteria adhesion to fish mucus is a crucial virulence mechanism. As the initial step of bacterial infection, adhesion is impacted by bacterial motility and environmental conditions. However, its molecular mechanism is yet unclear. In this study, a significant decrease in gene expression of adhesion-deficient Vibrio harveyi was observed when the bacteria were subjected by Cu2+(50 mg/L), Pb2+(100 mg/L), Hg2+(25 mg/L), and Zn2+(50 mg/L). The genes fliA , fliR , and flrB were responsible for flagellation; being crucial for adhesion, these genes were identified and silenced via RNAi. After silencing of these genes by RNAi technology, the ability of adhesion, biofilm formation, motility, and flagella synthesis of V. harveyi were considerably reduced. Compared with the control group, it was observed that the expression levels of fliS , fliD , flgH , and flrC were significant down-regulated in fliR -RNAi, flrB -RNAi, and fliA -RNAi. This data indicates that the expression levels of most virulence genes are affected by fliA , fliR , and flrB. Also, the expression of fliA , fliR , and flrB can be influenced by the salinity, temperature, and pH. The results show that: (1) fliA , fliR , and flrB have important roles in the adhesion of V. harveyi ; (2) fliA , fliR , and flrB can regulate bacterial adhesion by affecting its motility, and biofilm formation; (3) fliA , fliR , and flrB can regulate adhesion ability of V. harveyi in different environments. [ABSTRACT FROM AUTHOR]

As an important pathogen in aquaculture, Pseudomonas plecoglossicida has caused heavy losses. The expression of an ABC transporter gene- L321_23611 of P. plecoglossicida at 18 °C was found significant higher than those at 28 °C by RNA-seq and qRT-PCR. RNAi significantly reduced the content of L321_ 23611 mRNA in P. plecoglossicida with a maximal decrease of 89.2%. Compared with the wild type strain, the infection of L321_23611- RNAi strain resulted in the reduction in mortality and the onset time delay of a kind of marine teleosts, Epinephelus coioides. The results of dual RNA-seq showed that the RNAi of L321_23611 resulted in a significant change in both pathogen and host transcriptome in the spleens of infected E. coioides. The result of GO and KEGG analysis from dual RNA-seq data showed both host genes of chemokine signaling pathway, coagulation and complement system, hematopoietic cell lineage pathway as well as hemoglobin complex GO term and pathogenic genes of bacterial-type flagellum-dependent cell mortality GO term and flagellar assembly, biosynthesis of amino acids and lysine biosynthesis systems pathways were mainly affected by L321_23611 gene of P. plecoglossicida. The results indicated that: 1. ABC transporter gene- L321_23611 was a virulent gene of P. plecoglossicida. 2. Both the activation of the host immune pathways and depression of pathogenic virulence-related pathways facilitated E. coioides to remove L321_23611 -RNAi strain than the wild type strain of P. plecoglossicida. • RNAi significantly reduced the content of L321_ 23611 mRNA of P. plecoglossicida. • RNAi of L321_23611 reduced the pathogenicity of P. plecoglossicida to E. coioides. • RNAi of L321_23611 significantly up-regulated host immune pathways. • RNAi of L321_23611 down-regulated some pathogenic genes of P. plecoglossicida. [ABSTRACT FROM AUTHOR]