Flagellar structures have been shown to participate in virulence in a variety of intestinal pathogens. Here, we have identified two potential flagellar genes of Listeria 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 of fliF and fliI appears to be downregulated at 37°C, like that of flaA, encoding flagellin. By constructing two chromosomal deletion mutants, we show that inactivation of either fliF or fliI (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 ⌬fliF and ⌬fliI 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 of L. 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 of L. monocytogenes.Listeria monocytogenes is a motile, facultative intracellular bacterium that causes food-borne infections in humans and animals, with symptoms of septicemia, meningitis, and meningo-encephalitis. This gram-positive bacterium is also widely distributed in the environment and is able to grow over a wide range of temperatures, pHs, and osmotic pressures (43). In animal models, bacteria have been shown to cross the gastrointestinal barrier and possibly penetrate the intestinal epithelial cells overlaying Peyer's patches (34,35). The organism then disseminates to the brain and to the spleen, liver, and other lymphatic systems (26). The virulence of L. monocytogenes is due to its capacity to invade and multiply within host cells, including macrophages and hepatocytes, as well as epithelial, endothelial, and neuronal cells. Each step of the infectious process is dependent on the production of virulence factors, including invasion proteins (InlA and InlB), listeriolysin O, phospholipases, and ActA, which are controlled by the pleiotropic transcriptional activator PrfA (11). The initial step of the internalization of L. monocytogenes into nonphagocytic cells involves an adhesion mediated in particular by internalin (InlA) and InlB surface proteins. InlA promotes entry of L. monocytogenes into human epithelial cells via E-cadherin as a receptor (30). InlB is involved in entry of the bacterium into a broad range of cell lines including hepatocytes and nonepithelial cells. InlB interacts with the receptor for the complement factor C1q, hepatocyte growth factor (c-Met), and glycosamine glycans (4). Recent findings suggest that ot...

Molecular chaperones play an essential role in the folding of nascent chain polypeptides, as well as in the refolding and degradation of misfolded or aggregated proteins. They also assist in protein translocation and participate in stress functions. We identified a gene, designated tig, encoding a protein homologous to trigger factor (TF), a cytosolic ribosome-associated chaperone, in the genome of Listeria monocytogenes. We constructed a chromosomal ⌬tig deletion and evaluated the impact of the mutation on bacterial growth in broth under various stress conditions and on pathogenesis. The ⌬tig deletion did not affect cell viability but impaired survival in the presence of heat and ethanol stresses. We also identified the ffh gene, encoding a protein homologous to the SRP54 eukaryotic component of the signal recognition particle. However, a ⌬ffh deletion was not tolerated, suggesting that Ffh is essential, as it is in Bacillus subtilis and Escherichia coli. Thus, although dispensable for growth, TF is involved in the stress response of L. monocytogenes. The ⌬tig mutant showed no or very modest intracellular survival defects in eukaryotic cells. However, in vivo it showed a reduced capacity to persist in the spleens and livers of infected mice, revealing that TF has a role in the pathogenicity of L. monocytogenes.

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