Functional Type 1 Secretion System Involved in Legionella pneumophila Virulence

Legionella pneumophila is a Gram-negative pathogen found mainly in water, either in a free-living form or within infected pro-tozoans, where it replicates. This bacterium can also infect humans by inhalation of contaminated aerosols, causing a severe form of pneumonia called legionellosis or Legionnaires ’ disease. The involvement of type II and IV secretion systems in the viru-lence of L. pneumophila is now well documented. Despite bioinformatic studies showing that a type I secretion system (T1SS) could be present in this pathogen, the functionality of this system based on the LssB, LssD, and TolC proteins has never been established. Here, we report the demonstration of the functionality of the T1SS, as well as its role in the infectious cycle of L. pneumophila. Using deletion mutants and fusion proteins, we demonstrated that the repeats-in-toxin protein RtxA is secreted through an LssB-LssD-TolC-dependent mechanism. Moreover, fluorescence monitoring and confocal microscopy showed that this T1SS is required for entry into the host cell, although it seems dispensable to the intracellular cycle. Together, these results underline the active participation of L. pneumophila, via its T1SS, in its internalization into host cells. Legionella pneumophila is a Gram-negative pathogen that colo-nizes aquatic environments, where it survives by infecting wa-ter protozoans, especially amoebas. Pathogenic strains of L. pneu-mophila emerge from the environment after replication in such protozoans, are disseminated through aerosols of contaminated water, and reach human alveolar macrophages, where they begin