Shigella flexneri is a Gram-negative bacterial pathogen that causes human bacillary dysentery resulting in over a million deaths annually worldwide. The pathogenicity of Shigella sp. is dependent on a complex macromolecular machine, the type 3 secretion system (T3SS), that delivers into host cells a set of effector proteins required for invasion. The Shigella sp. T3SS consists of structural components of the injection machinery, secreted proteins, chaperones and regulators, all of which are encoded by approximately 25 genes located in the mxi, spa and ipa operons on a large 230 kb plasmid.1–3