Divergent Regulation of Virulence Factors by FimK in Classical and Hypervirulent Klebsiella pneumoniae Pathotypes.

Klebsiella pneumoniae is an opportunistic bacterial pathogen that is increasingly recognized as a major global threat. Type 1 pili, encoded by the fim operon, are virulence factors imperative in K. pneumoniae urinary tract infection. FimK, a regulatory protein, has been shown to repress type 1 fimbrial expression in specific K. pneumoniae strains, but its role across diverse K. pneumoniae pathotypes has not been fully elucidated. In this study, we investigate the influence of fimK on key virulence factors and phenotypes, including pili, capsule, and hypermucoviscosity (HMV), across five classical K. pneumoniae (cKp) and five hypervirulent K. pneumoniae (hvKp) strains. Furthermore, we disrupted the catalytic EAL domain (E245A) of chromosomal FimK in the cKp strain TOP52 to determine the specific contribution of its enzymatic activity to virulence. Our data reveal a striking divergence in FimK-mediated regulation between pathotypes. In cKp strains, excluding TOP52, fimK deletion did not significantly alter the capsule or HMV levels; in TOP52 fimK deletion increased FimA production ∼10-fold and reduced capsule and HMV levels. Conversely, while hvKp capsule levels remained relatively stable, fimK deletion led to variable but significant effects on HMV. Interestingly, the E245A mutation in TOP52 FimK resulted in an approximately 2-log reduction in lung bacteria burden at 24 hours, mirroring the effects of a full gene deletion. These findings demonstrate that FimK regulation of type 1 pili and its broader role in virulence are highly strain-specific and dependent on its catalytic activity in strain TOP52.