A Siderophore (greek for iron carrier) is an iron chelating compound secreted by microorganisms. Iron Fe3+ ions have a very low solubility at neutral pH and therefore cannot be utilized by organisms. Siderophores dissolve these ions as soluble Fe3+ complexes that can be taken up by active transport mechanisms. Many siderophores are nonribosomal peptides.
Other strategies to enhance iron solubility and uptake are the acidification of the surrounding (e.g. used by plant roots) or the extracellular reduction of Fe3+ into the more soluble Fe2+ ions.
Examples of siderophores produced by various bacteria and fungi are ferrichrome (Ustilago sphaerogena), enterobactin (Escherichia coli), enterobactin and bacillibactin (Bacillus subtilis), ferrioxamine B (Streptomyces pilosus), fusarinine C (Fusarium roseum), yersiniabactin (Yersinia pestis), vibriobactin (Vibrio cholerae), or pseudobactin (Pseudomonas B 10).