Flagellum of Legionella pneumophila positively affects the early phase of infection of eukaryotic host cells

Legionella pneumophila, the etiologic agent of Legionnaires' disease, conta ins a single, monopolar flagellum which is composed of one major subunit, t he FlaA protein. To evaluate the role of the flagellum in the pathogenesis and ecology of Legionella, the flaA gene of L. pneumophila Corby was mutage nized by introduction of a kanamycin resistance cassette. Immunoblots with antiflagellin-specific polyclonal antiserum, electron microscopy, and motil ity assays confirmed that the specific flagellar mutant L. pneumophila Corb y KH3 was nonflagellated. The redelivery of the intact flaA gene into the c hromosome (L. pneumophila Corby CD10) completely restored flagellation and motility. Coculture studies showed that the invasion efficiency of the flaA mutant was moderately reduced in amoebae and severely reduced in HL-60 cel ls. In contrast, adhesion and the intracellular rate of replication remaine d unaffected. Taking these results together, we have demonstrated that the flagellum of L. pneumophila positively affects the establishment of infecti on by facilitating the encounter of the host cell as well as by enhancing t he invasion capacity.