FimA mediates adhesion, colonization and inflammatory injury in bovine mastitis caused by Klebsiella pneumoniae

Bovine mastitis caused by Klebsiella pneumoniae is an infectious disease characterized by severe clinical manifestations, poor therapeutic outcomes, and substantial economic losses. Although type 1 fimbriae, with fimA as the major structural subunit, are a recognized virulence factor in Enterobacteriaceae, their specific role in bovine mastitis remains unclear. In this study, we constructed a ΔfimA mutant from a clinical mastitis isolate to evaluate contributions of fimA to adhesion and virulence. Transmission electron microscopy revealed impaired fimbrial assembly in the mutant, whereas adhesion assays demonstrated significantly reduced binding to bovine mammary epithelial cells in vitro. At the bacterial level, transcriptome analysis revealed downregulation of type 1 fimbrial genes, iron acquisition systems, outer membrane protein, and several stress-related proteins, whereas type 3 fimbrial genes and lysozyme inhibitors were upregulated, suggesting compensatory regulation of adhesion and stress responses. In an in vivo murine intramammary infection model, ΔfimA infection, compared with wild type (WT), resulted in significantly lower bacterial load, attenuated mammary inflammation, reduced histopathological damage, and decreased cytokine concentrations. Specifically, TNF-α and IL-6 were significantly reduced in ΔfimA-infected tissues, Gasdermin D (GSDMD) cleavage was significantly lower than in WT, and IL-1β and NLRP3 had downward but nonsignificant trends. In addition, quantitative real-time PCR confirmed differential expression of representative immune-related genes, supporting a weaker proinflammatory response in the absence of fimA. Collectively, these findings implicated fimA as a key determinant of fimbrial assembly, epithelial adhesion and proinflammatory activation in the mammary gland, providing new mechanistic insights into relationships among fimbriae, colonization, and virulence in K. pneumoniae.