Role of FliF and FliI of Listeria monocytogenes in Flagellar Assembly and Pathogenicity

Flagellar structures have been shown to participate in virulence in a variety of intestinal pathogens. Here, we have identified two potential flagellar genes ofListeria monocytogenes : lmo0713, encoding a protein similar to the flagellar basal body component FliF, and lmo0716, encoding a protein similar to FliI, the cognate ATPase energizing the flagellar export apparatus. Expression offliF andfliI appears to be downregulated at 37°C, like that offlaA , encoding flagellin. By constructing two chromosomal deletion mutants, we show that inactivation of eitherfliF orfliI (i) abolishes bacterial motility and flagella production, (ii) impairs adhesion and entry into nonphagocytic epithelial cells, and (iii) also reduces uptake by bone marrow-derived macrophages. However, the ΔfliF and ΔfliI mutations have only a minor impact on bacterial virulence in the mouse model, indicating that the flagellar secretion apparatus itself is not essential for survival in this animal model. Finally, among 100 human clinical isolates ofL. monocytogenes tested, we found 20 strains still motile at 37°C. Notably, all these strains adhered less efficiently than strain EGD-e to Caco-2 cells at 37°C but showed no defect of intracellular multiplication. These data suggest that expression of the flagella at 37°C might hinder optimal adhesion to epithelial cells but has no impact on intracytosolic survival ofL. monocytogenes .