Enteric pathogens express a variety of virulence factors associated with the production of disease.
One type of virulence factor is the expression of proteins associated with the acquisition of the essential element iron. Because there is little free iron in mammalian hosts, pathogenic bacteria must express high-affinity iron acquisition systems to establish and maintain an infection. These include receptors and transport systems for host iron complexes and the synthesis and transport of siderophores, low molecular weight iron-binding compounds that can remove iron from host proteins. The Shigella species have at least three distinct iron transport systems. Two of these, the aerobactin and the enterobactin systems, consist of siderophores and their associated outer membrane receptor proteins.