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 Klebsiella pneumoniae, a notable opportunistic pathogen associated with hospital-acquired infections. Our transcriptional analyses revealed that deleting the fis gene caused a significant upregulation of mrkA and mrkH, the genes responsible for the structure and regulation of type 3 fimbriae, respectively. Additionally, phenotypic assays demonstrated that the Δfis 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 cis-complemented strain. Electrophoretic mobility shift assays confirmed that Fis directly binds to the regulatory regions upstream of both mrkA and mrkH, indicating that repression occurs through direct interaction with the promoter. In summary, our findings show that Fis acts as a transcriptional repressor of mrkA and mrkH, 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 K. pneumoniae, deepening our understanding of its virulence regulatory network.