Figure 2.

Shigella infectious cycle: interaction of T3SA with membrane compartments is key for the regulation of its activity. The “invade and evade” infectious strategy of S. flexneri can be broken down in two phases: (1) entry, characterized by residence of bacteria in vacuoles derived from the PM (1a,b), which are ultimately ruptured (1c); (2) cytoplasmic residence, where most replication events occur (2a) and motility through actin comet formation is possible (2b). After bacteria have reached the cytoplasm, they are in position to iterate this cycle and progressively invade neighboring cells, before evading once again the secondary vacuole. This process is characterized by the formation of protrusions (3a) and vacuoles (3b) composed of a double membrane derived from the PM in which bacteria reside until their lysis (3c), and escape in the cytoplasm (2^*). It was demonstrated that secreting bacteria (green) were systematically associated with entry and cell-to-cell spread vacuoles and protrusions derived from the PM, while cytoplasmic bacteria were not actively secreting (gray) (A). Magnification of the inner and outer leaflet of the PM. Density of cholesterol (yellow rectangles) and overall phospholipids composition (pink vs. blue) is variable in both leaflets. Therefore, bacteria are not facing the same biochemical cues when they are performing entry vs. cell-to-cell spread. As mentioned in panel (A), bacteria also face four membranes during cell-to-cell spread instead of two during entry (B). Proposed mechanisms of inactivation of T3SA in intracellular Shigella. Grey circles and white rectangles represent secreted tip complex proteins, which are incapable of blocking T3SA conduit. H1 and H2 represent alternative hypotheses for inactivation of T3SA in the cytoplasm of host cells, as described in the text. H1: replenishment of functional tip complex; H2: disassembly of T3SA before vacuole escape and replenishment with inactive T3SA in the host cytoplasm (C).