Mechanisms underlying the virulence regulation of Aeromonas salmonicida SRW-OG1 fhuB gene revealed by an iron-limited transcriptomic analysis

Iron is an indispensable micronutrient for bacterial survival and pathogenicity . The purpose of this study was to investigate how iron limitation affects virulence regulation in an important pathogenic Aeromonas salmonicida SRW-OG1, focusing on the molecular mechanism of the fhuB gene, which regulates the virulence of A. salmonicida SRW-OG1. Transcriptome sequencing was performed on A. salmonicida SRW-OG1 cultured under iron-limited conditions, and Real-time quantitative PCR (RT-qPCR) analysis related virulence phenotypes used for validation. The highly expressed fhuB gene, involved in Fe 3 + -hydroxamate siderophores transport system, was identified from the transcriptomic data, and Δ fhuB mutant strain was constructed using homologous recombination technology. The results showed that 1409 differentially expressed genes (DEGs) (988 up-regulated and 577 down-regulated) were identified in A. salmonicida SRW-OG1 under iron-limited conditions. The main enriched Gene Ontology (GO) terms included metabolic process, membrane and transporter activity. The major Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways included ribosome and biosynthesis of siderophore group nonribosomal peptides. Interestingly, iron limitation significantly enhanced extracellular enzyme activity, as well as biofilm formation, chemotaxis, and motility in A. salmonicida SRW-OG1. The Δ fhuB strain showed significant reductions in these virulence-related phenotypes under iron-limited conditions, including decreased motility (18.7%), adhesion (17.6%), and chemotaxis (43.2%). In addition, compared with SRW-OG1, the cumulative mortality of Epinephelus coioides (orange-spotted grouper) infected with the Δ fhuB strain decreased by 60%, indicating a significant protective effect in E. coioides . Our research demonstrates that A. salmonicida adapts to iron limitation by globally reprogramming its transcriptome and activating the FhuB-mediated siderophore transport system. Iron influences various aspects of virulence in A. salmonicida SRW-OG1, and the gene fhuB plays a critical role in regulating virulence in A. salmonicida SRW-OG1 under iron-limited conditions. This study provides mechanistic insights into iron-dependent pathogenesis and identifies potential targets for disease control in aquaculture.