Formation of a fibrous structure on the surface of Legionella pneumophila associated with exposure of DotH and DotO proteins after intracellular growth

Legionella pneumophila grows in human alveolar macrophages and resides with in a phagosome that initially lacks proteins associated with the endocytic pathway. Required for targeting to this unique location is the Dot/Icm comp lex, which is highly similar to conjugative DNA transfer apparatuses. Here, we show that exposure to three distinct inducing conditions resulted in th e formation of a fibrous structure on the bacterial cell surface that conta ined the DotH and DotO proteins. These conditions included: (i) incubation for 2 h with mouse bone marrow-derived macrophages; (ii) incubation for 2 h in macrophage-conditioned media; or (iii) replication of bacteria for 22 h within macrophages. Introduction of bacteria harbouring the surface-expose d DotH and DotO onto a fresh monolayer resulted in loss of the surface loca lization of DotH and DotO shortly after uptake. Treatments that resulted in the production of the fibrous structure enhanced the rate at which the bac teria were internalized, but there was no corresponding increase in the eff iciency of intracellular growth compared with bacteria that had been cultur ed in broth using conditions that resulted in maximal intracellular growth. These data indicate that the surface-exposed DotH and DotO on L. pneumophi la may act either just before lysis from the macrophage or at the earliest stages of infection, transiently relocating in a fibrous structure on the b acterial cell surface.