Role of flagellin and the two-component CheA/CheY system of Listeria monocytogenes in host cell invasion and virulence.

The flagellum protein flagellin of Listeria monocytogenes is encoded by the flaA gene. Immediately downstream of flaA, two genes, cheY and cheA, encoding products with homology to chemotaxis proteins of other bacteria, are located. In this study we constructed deletion mutants with mutations in flaA. cheY, and cheA to elucidate their role in the biology of infection with L. monocytogenes. The DeltacheY, DeltacheA, and double-mutant DeltacheYA mutants, but not DeltaflaA mutant, were motile in liquid media. However, the DeltacheA mutant had impaired swarming and the DeltacheY and DeltacheYA mutants were unable to swarm on soft agar plates, suggesting that cheY and cheA genes encode proteins involved in chemotaxis. The DeltaflaA, DeltacheY, DeltacheA, and DeltacheYA mutants (grown at 24 degrees C) showed reduced association with and invasion of Caco-2 cells compared to the wild-type strain. However, spleens from intragastrically infected BALB/c and C57BL/6 mice showed larger and similar numbers of the DeltaflaA and DeltacheYA mutants, respectively, compared to the wild-type controls. Such a discrepancy could be explained by the fact that tumor necrosis factor receptor p55 deficient mice showed dramatically exacerbated susceptibility to the wild-type but unchanged or only slightly increased levels of the DeltaflaA or DeltacheYA mutant. In summary, we show that listerial flaA. cheY, and cheA gene products facilitate the initial contact with epithelial cells and contribute to effective invasion but that flaA could also be involved in the triggering of immune responses.