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Abstract: Edwardsiella tarda is a Gram-negative bacterium that can infect a wide variety of marine and freshwater fish and cause severe economic losses worldwide. With an aim to elucidate the virulence mechanism of E. tarda, we in this study cloned and analyzed the function of an iron-cofactored superoxide dismutase (FeSOD) from a pathogenic E. tarda strain TX01 isolated from diseased fish. FeSOD is 192-residue in length and contains domain structures that are conserved among iron/manganese superoxide dismutases. Recombinant FeSOD purified from Escherichia coli exhibits apparent superoxide dismutase activity. Quantitative real-time RT-PCR analysis indicated that FeSOD expression is significantly upregulated immediately following TX01 infection of Japanese flounder (Paralichthys olivaceus) head kidney (HK) macrophages and cultured FG cells. Compared to the wild type strain TX01, the FeSOD mutant strain TXSod is (i) more sensitive to H2O2-induced oxidative damage, (ii) less resistant against serum- and macrophage-mediated bacterial killing, (iii) significantly weakened in the ability to invade into FG cells and to disseminate in fish blood and liver, and (iv) deficient in blocking macrophage respiratory burst activity and production of reactive oxygen species. Furthermore, HK macrophages infected by TXSod exhibits significantly increased expression of inflammatory cytokines compared to macrophages infected by TX01. Taken together, these results indicate that FeSOD is a virulence factor that plays an important role in the pathogenicity of E. tarda by inhibiting macrophage-mediated innate immune response. [ABSTRACT FROM AUTHOR]

MicroRNAs (miRNAs) are involved in many biological activities including immune defense against pathogens. In this study, we applied high-throughput sequencing technology to examine miRNAs in Japanese flounder (Paralichthys olivaceus) infected with Streptococcus iniae at different times. A total of 1038 miRNAs were identified, of which, 249 were novel miRNAs, and 81 showed differential expression (named DEmiRNAs) after S. iniae infection. Of the 81 DEmiRNAs identified, 34 and 58 occurred at 6 h and 24 h post-infection, respectively; most DEmiRNAs were strongly time-specific, and only 13.6% of the DEmiRNAs were shared between the two time points. A total of 9582 target genes were predicted for the 81 DEmiRNAs. The putative target genes were enriched in various GO and KEGG pathways of biological processes and molecular/cellular functions, in particular endocytosis, regulation of transcription, lysososme, and the signaling pathways of MAPK, ErbB, and AMPK. One of the DEmiRNAs, pol-3p-10740_175, was found to target dual specificity phosphatase 6 (Dusp6) and repress the expression of the latter. Transfection of flounder FG cells with pol-3p-10740_175 caused a significant inhibition on S. iniae invasion. The results of this study provided the first S. iniae -induced miRNA profile in Japanese flounder and indicated that flounder miRNAs play an important role in antibacterial immunity. • Streptococcus iniae infection altered the expression of 81 miRNAs in flounder. • There were 34 and 58 differentially expressed miRNAs at 6 and 24 hpi, respectively. • The putative target genes of the miRNAs were enriched in various GO/KEGG pathways. • One miRNA, pol-3p-10740_175, negatively regulated dual specificity phosphatase 6. • Overexpression of pol-3p-10740_175 significantly inhibited S. iniae invasion. [ABSTRACT FROM AUTHOR]