Our laboratory is interested in understanding the cellular and molecular mechanisms that allow microbial pathogens to survive and multiply within the hostile host cells. We use Legionella pneumophila, the causative agent of Legionnaires’ disease as a model organism. This bacterium is a facultative intracellular pathogen capable of growing in a vacuole within macrophages as well as fresh water amoebae. After uptake, the Legionella-containing vacuole (LCV) in its early phase evades fusion with the lysosomal network and later is transformed into a compartment with characteristics of rough endoplasmic reticulum. Essential to the biogenesis of this replicative vacuole is the Dot/Icm Type IV protein secretion system that injects a large number of effector proteins into target host cells. These effectors modulates a variety of host cellular processes, such as remodeling of phagosomal membrane, vesicular trafficking, cell death, stress response thus allowing the establishment of the replicative compartment that supports bacterial growth. Our current focus is to analyze biochemical and cell biological activities conferred by these proteins and their roles in promoting the unique trafficking of the Legionella-containing vacuole in phagocytic cells. In particular, we are interested in identification of host proteins whose activities are modulated by substrates of the Dot/Icm system and the roles of such modulation in intracellular bacterial growth.