Vibrio harveyi is ubiquitous in warm seawater and sediments and has become a common bacterial pathogen isolated from aquatic animals and even humans (Brehm et al., 2020;Joana et al., 2019;Peeralil et al., 2020). It is also a member of intestinal microflora in the intestines of marine vertebrates and invertebrates (Rungrassamee et al., 2016). Vibrio harveyi mainly infects shrimp (Martin et al., 2004;Zhou et al., 2012) and causes intestinal tract (Rungrassamee et al., 2016) and hepatopancreatic necrosis (Sarmila et al., 2019). Under natural conditions, fish are generally more susceptible to V. harveyi than invertebrates (Owens & Busico-Salcedo, 2006). Vibrio harveyi generally infect commercially important fish such as orange-spotted grouper (Epinephelus coioides) (Bai et al., 2020) and Atlantic salmon (Salmo salar) (Zhang & Austin, 2000). It also causes muscular ulcer, tail erosion, eye infection, symptoms of tongue sole (Cynoglossus semilaevis) and gilthead bream (Sparus aurata) (Haldar et al., 2010;Li et al., 2019), and 'Scale Drop and Muscle Necrosis disease' (SDMND)

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Vibrio harveyi is commonly found in salt and brackish water, and is recognized as a serious bacterial pathogen in aquaculture worldwide. In this study, we cloned the ferric uptake regulator (Fur) gene from V. harveyi wild-type strain HA_1, which was isolated from diseased American eels (Anguilla rostrata) and has a length of 450 bp, encoding 149 amino acids. Then, a mutant strain, HA_1-ΔFur, was constructed through homologous recombination of a suicide plasmid (pCVD442). The HA_1-ΔFur mutant exhibited attenuated biofilm formation, intensified swarming motility, and 18-fold decrease (5.5%) in virulence to the American eels, but it showing no difference in growth and hemolysis with the wild-type strain. Transcriptome analysis revealed that 875 genes were differentially expressed in the ΔFur mutant, with 385 up-regulated and 490 down-regulated DEGs. GO and KEGG enrichment analysis revealed that, compared to the wild-type strain, the type II secretion systems (T2SS), type VI secretion systems (T6SS), amino acid synthesis and transport, and energy metabolism pathways were significantly down-regulated, but the ABC transporters and biosynthesis of siderophore group non-ribosomal peptides pathways were up-regulated in the ΔFur strain. The qRT-PCR results further confirmed that DEGs responsible for amino acid transport and energy metabolism were positively regulated, but DEGs involved in iron acquisition were negatively regulated in the ΔFur strain. These findings suggest that the gene Fur contribute to the virulence of V. harveyi through biofilm formation, energy metabolism, and transcript regulation.