Functional Type 1 Secretion System Involved in Legionella pneumophila Virulence

International audienceLegionella pneumophilais 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 severeform of pneumonia called legionellosis or Legionnaires’ disease. The involvement of type II and IV secretion systems in the viru-lence ofL. pneumophilais 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 beenestablished. Here, we report the demonstration of the functionality of the T1SS, as well as its role in the infectious cycle ofL.pneumophila. Using deletion mutants and fusion proteins, we demonstrated that the repeats-in-toxin protein RtxA is secretedthrough an LssB-LssD-TolC-dependent mechanism. Moreover, fluorescence monitoring and confocal microscopy showed thatthis T1SS is required for entry into the host cell, although it seems dispensable to the intracellular cycle. Together, these resultsunderline the active participation ofL. pneumophila, via its T1SS, in its internalization into host cells