Genetics and molecular biology of siderophore-mediated iron transport in bacteria

The possession of specialized iron transport systems may be crucial for bacteria to override the iron limitation imposed by the host or the environment. One of the most commonly found strategies evolved by microorganisms is the production of siderophores, low-molecular-weight iron chelators that have very high constants of association for their complexes with iron. Thus, siderophores act as extracellular solubilizing agents for iron from minerals or organic compounds, such as transferrin and lactoferrin in the host vertebrate, under conditions of iron limitation. Transport of iron into the cell cytosol is mediated by specific membrane receptor and transport systems which recognize the iron-siderophore complexes. In this review I have analyzed in detail three siderophore-mediated iron uptake systems: the plasmid-encoded anguibactin system of Vibrio anguillarum, the aerobactin-mediated iron assimilation system present in the pColV-K30 plasmid and in the chromosomes of many enteric bacteria, and the chromosomally encoded enterobactin iron uptake system, found in Escherichia coli, Shigella spp., Salmonella spp., and other members of the family Enterobacteriaceae. The siderophore systems encoded by Pseudomonas aeruginosa, namely, pyochelin and pyoverdin, as well as the siderophore amonabactin, specified by Aeromonas hydrophila, are also discussed. The potential role of siderophore-mediated systems as virulence determinants in the specific host-bacteria interaction leading to disease is also analyzed with respect to the influence of these systems in the expression of other factors, such as toxins, in the bacterial virulence repertoire.