ABSTRACTLegionella pneumophilais the causative agent of a pneumonia termed Legionnaires’ disease. The facultative intracellular bacterium employs the Icm/Dot type IV secretion system (T4SS) and a plethora of translocated “effector” proteins to interfere with host vesicle trafficking pathways and establish a replicative niche, theLegionella-containing vacuole (LCV). Internalization of the pathogen and the events immediately ensuing are accompanied by host cell-mediated phosphoinositide (PI) lipid changes and the Icm/Dot-controlled conversion of the LCV from a PtdIns(3)P-positive vacuole into a PtdIns(4)P-positive replication-permissive compartment, which tightly associates with the endoplasmic reticulum. The source and formation of PtdIns(4)Pare ill-defined. Using dually labeledDictyostelium discoideumamoebae and real-time high-resolution confocal laser scanning microscopy (CLSM), we show here that nascent LCVs continuously capture and accumulate PtdIns(4)P-positive vesicles from the host cell. Trafficking of these PtdIns(4)P-positive vesicles to LCVs occurs independently of the Icm/Dot system, but their sustained association requires a functional T4SS. During the infection, PtdIns(3)P-positive membranes become compacted and segregated from the LCV, and PtdIns(3)P-positive vesicles traffic to the LCV but do not fuse. Moreover, using eukaryotic and prokaryotic PtdIns(4)Pprobes (2×PHFAPP-green fluorescent protein [2×PHFAPP-GFP] and P4CSidC-GFP, respectively) along with Arf1-GFP, we show that PtdIns(4)P-rich membranes of thetrans-Golgi network associate with the LCV. Intriguingly, the interaction dynamics of 2×PHFAPP-GFP and P4CSidC-GFP are spatially separable and reveal the specific PtdIns(4)Ppool from which the LCV PI originates. These findings provide high-resolution real-time insights into howL. pneumophilaexploits the cellular dynamics of membrane-bound PtdIns(4)Pfor LCV formation.IMPORTANCEThe environmental bacteriumLegionella pneumophilacauses a life-threatening pneumonia termed Legionnaires’ disease. The bacteria grow intracellularly in free-living amoebae as well as in respiratory tract macrophages. To this end,L. pneumophilaforms a distinct membrane-bound compartment called theLegionella-containing vacuole (LCV). Phosphoinositide (PI) lipids are crucial regulators of the identity and dynamics of host cell organelles. The PI lipid PtdIns(4)Pis a hallmark of the host cell secretory pathway, and decoration of LCVs with this PI is required for pathogen vacuole maturation. The source, dynamics, and mode of accumulation of PtdIns(4)Pon LCVs are largely unknown. UsingDictyosteliumamoebae producing different fluorescent probes as host cells, we show here that LCVs rapidly acquire PtdIns(4)Pthrough the continuous interaction with PtdIns(4)P-positive host vesicles derived from the Golgi apparatus. Thus, the PI lipid pattern of the secretory pathway contributes to the formation of the replication-permissive pathogen compartment.