The Nucleoid-Associated Protein Fis Represses Type 3 Fimbriae to Modulate Biofilm and Adherence Formation in Klebsiella pneumoniae

The nucleoid-associated protein Fis functions as a global regulator that influences various cellular processes in Gram-negative bacteria. In this study, we examined the role of Fis in the transcriptional regulation of type 3 fimbriae in <i>Klebsiella pneumoniae</i>, a notable opportunistic pathogen associated with hospital-acquired infections. Our transcriptional analyses revealed that deleting the <i>fis</i> gene caused a significant upregulation of <i>mrkA</i> and <i>mrkH</i>, the genes responsible for the structure and regulation of type 3 fimbriae, respectively. Additionally, phenotypic assays demonstrated that the Δ<i>fis</i> mutant exhibited enhanced biofilm formation and greater adherence to A549 lung epithelial cells compared to the wild-type strain. These effects were restored to wild-type levels in the <i>cis</i>-complemented strain. Electrophoretic mobility shift assays confirmed that Fis directly binds to the regulatory regions upstream of both <i>mrkA</i> and <i>mrkH</i>, indicating that repression occurs through direct interaction with the promoter. In summary, our findings show that Fis acts as a transcriptional repressor of <i>mrkA</i> and <i>mrkH</i>, thereby negatively regulating the expression of type 3 fimbriae, biofilm formation, and adherence. This study highlights Fis as a direct regulator of fimbrial expression and biofilm development in <i>K. pneumoniae</i>, deepening our understanding of its virulence regulatory network.