Intracellular bacteria have developed various mechanisms to enter and persist in host cells and, at the same time, to evade the host immune response. One such pathogen is Francisella tularensis, the etiological agent of tularemia. After phagocytosis, this Gram-negative bacterium quickly escapes from the phagocytic compartment and replicates in the host cell cytosol. For this mode of infection, several components of the Francisella pathogenicity island (FPI) are critical. Interestingly, some FPI proteins share homology to components of Type VI Secretion Systems (T6SSs), but their assembly and functionality remains to be shown in Francisella.The thesis focused on the characterization of several of these FPI components; more specifically, how they contribute to the infection cycle as well as their possible role in the putative T6SS. We identified three unique mutants, ΔiglG, ΔiglI and ΔpdpE, which to various degrees were able to escape the phagosomal compartment, replicate in the host cytosol and cause host cell cytotoxicity. In contrast, ΔiglE as well as mutants within the conserved core components of T6SSs, VgrG and DotU, were defective for all of these processes. In the case of IglE, which is a lipoprotein and localized to the outer membrane of the bacterial cell wall, residues within its N-terminus were identified to be important for IglE function. Consistent with a suggested role as a trimeric membrane puncturing device, VgrG was found to form multimers. DotU stabilized the inner membrane protein IcmF, in agreement with its function as a core T6SS component. The functionality of the secretion system was shown by the translocation of several FPI proteins into the cytosol of infected macrophages, among them IglE, IglC and VgrG, of which IglE was the most prominently secreted protein. At the same time, the secretion was dependent on the core components VgrG, DotU but also on IglG. Although we and others have shown the importance of FPI proteins for the escape of F. tularensis, it has been difficult to assess their role in the subsequent replication, since mutants that fail to escape never reach the growth-permissive cytosol. For this reason, selected FPI mutants were microinjected into the cytosol of different cell types and their growth compared to their replication upon normal uptake. Our data suggest that not only the metabolic adaptation to the cytosolic compartment is important for the replication of intracytosolic bacteria, but also the mechanism of their uptake as well as the permissiveness of the cytosolic compartment per se.