The cell biology of infection by intracellular bacterial pathogens.

Listeria monocytogenes and Shigella flexneri are unrelated bacterial pathogens that have independently evolved similar strategies of survival within an infected host animal. Bacteria coming into contact with the surface of an epithelial cell induce cytoskeletal rearrangements resulting in phagocytosis. They then secrete enzymes that degrade the phagosomal membrane, releasing the bacteria into the host cytoplasm. Intracytoplasmic bacteria move rapidly, in association with a "comet tail" made up of host cell actin filaments. When moving bacteria reach the cell margin, they push out long protrusions with the bacteria at the tips that are then taken up by neighboring cells, allowing the infection to spread from cell to cell. This review summarizes what is currently known about the interactions between the bacteria and the host at each stage of the infection and discusses what mammalian cell biologists can learn by studying bacterial pathogens.