A proposed model for invasion of epithelial cells of the colon.
1) The Shigella first cross the mucosa by passing through specialized cells called M cells. The M cell passes the Shigella on to a macrophage from which it subsequently escapes - possibly by inducing apoptosis, a programmed cell suicide.
2) The Shigella then uses its invasins to enter the mucosal epithelial cells from underneath. The invasins cause actin polymer rearrangements in the cell's cytoskeleton resulting in the bacterium being engulfed and placed in an endocytic vesicle in a manner similar to phagocytic cells. Once inside, the Shigella escape from the vacuole into the cytoplasm and multiply.
3) The Shigella are able to move through the host cell and spread to adjacent host cells by a unique process called actin-based motility. In this process, actin filaments polymerize at one end of the bacterium, producing comet-like tails that propel the Shigella through the cytoplasm of the host cell.
4) When they reach the boundary of that cell, the actin filaments push the Shigella across that membrane and into the adjacent cell.