Type III secretion: a bacterial device for close combat with cells of their eukaryotic host

Salmonella, Shigella, Yersinia, Pseudomonas aeruginosa, enteropathogenic Es cherichia coli and several plant-pathogenic Cram-negative bacteria use a ne w type of systems called 'type III secretion' to attack their host. These s ystems are activated by contact with a eukaryotic cell membrane and they al low bacteria to inject bacterial proteins across the two bacterial membrane s and the eukaryotic cell membrane to reach a given compartment and destroy or subvert the target cell. Those systems consist of a secretion apparatus made up of about 25 individual proteins and a set of proteins released by this apparatus. Some of these released proteins are 'effectors' that are de livered by extracellular bacteria into the cytosol of the target cell while the others are 'translocators' that help the 'effectors' to cross the memb rane of the eukaryotic cell. Most of the 'effectors' act on the cytoskeleto n or on intracellular signalling cascades. One of the proteins injected by the enteropathogenic E. coli serves as a membrane receptor for the docking of the bacterium itself at the surface of the cell.