Participation of the Transcriptional Regulator Fur in Modulating Biofilm Formation of the Salmonid Pathogen Yersinia ruckeri: Beyond Iron Homeostasis.

Biofilm formation by bacterial pathogens represents a major challenge in aquaculture, since biofilms promote persistence and increase tolerance to antimicrobial treatments. Yersinia ruckeri, the etiological agent of enteric redmouth disease in salmonids, contributes to recurrent infections in aquaculture. However, the molecular mechanisms governing biofilm development in this pathogen remain poorly understood. In this study, we investigated the role of the global transcriptional regulator Fur in the biofilm formation of Y. ruckeri. To this aim, a fur deletion mutant (Δfur) was phenotypically, structurally, and transcriptionally characterized under biofilm-forming conditions. Fur deletion resulted in impaired motility, reduced flagellar synthesis, and a marked decrease in mature biofilm formation. These changes were accompanied by alterations in three-dimensional architecture, cellular organization, and extracellular matrix production, as revealed by scanning electron and confocal microscopy analyses. Furthermore, transcriptomic profiling via RNA-seq demonstrated that the absence of Fur leads to extensive transcriptional reprogramming. Genes associated with the biosynthesis of flagella and oxidative stress response were significantly down-regulated, whereas genes associated with SOS responses, non-ribosomal peptide biosynthesis, and those related to iron acquisition and siderophore systems exhibited significant up-regulation in the absence of Fur. Interestingly, the strong up-regulation of P2-type prophage genes was also observed in the Δfur strain. Collectively, our findings identify Fur as an important contributor to the normal progression toward mature and spatially organized Y. ruckeri biofilms. Likewise, the Fur-dependent pathways and factors identified here provide candidates for future mechanistic studies and may ultimately help identify strategies to interfere with Y. ruckeri persistence in aquaculture environments.