Effects of the lpfD gene on biofilm formation in Salmonella

Salmonella biofilm (BF) formation is crucial for persistent infections, with fimbrial adhesion being key. The regulatory role of the lpfD gene, encoding the tip adhesin of long polar fimbriae (LPF), in BF development is not well understood. This study used whole-genome sequencing to identify the lpfD gene difference between high-BF-forming strain DSE06 and low-BF-forming strain DSK01. Molecular biology techniques created the lpfD knockout strain DSE06-ΔlpfD, complemented strain DSE06-CΔlpfD, and recombinant strain DSK01-lpfD(+). Growth curves and BF formation of these strains were analyzed using culturing, crystal violet staining, SEM, and TEM. Adhesion and invasion efficiencies on Caco-2 cells were compared, and mRNA expression of key BF genes csgD, csgA, and csgB was evaluated. Results showed genetic modifications did not influence bacterial growth. Among wild-type, knockout, complemented and recombinant strains, the BF-forming capacity of the DSE06-ΔlpfD was significantly reduced (P < 0.01), whereas the DSK01-lpfD(+) demonstrated a significant enhancement (P < 0.01), the DSE06-CΔlpfD exhibited a partial restoration. BF formed by the DSE06 and the DSK01-lpfD(+) strains displayed dense net structures, in contrast to the dispersed bacterial distribution in DSE06-ΔlpfD. The deletion of lpfD did not alter the ultrastructural morphology of LPF. Compared to the DSE06, DSE06-CΔlpfD, and DSK01-lpfD(+) strains, the DSE06-ΔlpfD strain generated showed significantly reduced adhesion and invasion rates. In the DSE06-ΔlpfD strain, the expression of csgD, csgA, and csgB was significantly reduced compared to the DSE06 strain (P < 0.01). These results highlight the lpfD gene's crucial role in Salmonella BF formation and its potential impact on controlling infections.