The role of type IV pilus retraction during the infection process of Neisseria gonorrhoeae

The early stage of infection with Neisseria gonorrhoeae, the causative agent of gonorrhea, is marked by type IV pilus (tfp)-mediated attachment and the formation of bacterial microcolonies on epithelial cells. An important feature of tfp is their ability to physically retract, mediated by the pilus retraction protein PilT. Retraction of the N. gonorrhoeae tfp generates substantial forces on its substrate such as on host cells and neighbouring bacteria. Previous work indicated that tfp retraction force elicits host cell signaling. In this thesis it was hypothesized that this retraction force could activate nuclear factor kappa B (NF- B), the central signaling cascade of innate immunity. Using a p65-GFP-expressing epithelial cell line it was shown that a retraction-deficient pilT mutant induces markedly reduced NF- B activation compared to piliated wild-type (wt) bacteria. This was accompanied by decreased NF- B target gene transcription and cytokine release. NF- B-activation levels induced by the pilT mutant were restored to wt-induced levels by applying mechanical shear stress to the infected host cells, indicating that the mechanical forces generated by retractile pili are involved in the retractiondependent activation of NF- B. Furthermore, using live cell microscopy, it was observed that piliated wt Neisseria induce asynchronous NF- B activation in infected cells, which is temporally associated with the formation of gonococcal microcolonies. Thus, this work provides evidence for a complex but crucial correlation between N. gonorrhoeae pilus retraction and associated forces, microcolony growth, and host cell signaling. Whether pilus retraction force also influences the interaction and communication between the bacteria themselves has not been investigated yet.