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    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 by chelation as soluble Fe3+ complex that can be taken up by active transport mechanisms. Many siderophores are nonribosomal peptides.
    To put it another way: Under anoxic conditions, iron is generally in the +2 oxidation state (ferrous) and soluble. However, under oxic conditions, iron is generally in the +3 oxidation state (ferric) and forms various insoluble minerals. To obtain iron from such minerals, cells produce iron-binding siderophores that bind iron and transport it into the cell. One major group of siderophores consists of derivatives of hydroxamic acid, which chelate ferric iron very strongly.Biology of Microorganisms, Eleventh Edition, Pearson Education
    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), mycobactin (Mycobacterium), enterobactin and bacillibactin (Bacillus subtilis), ferrioxamine B (Streptomyces pilosus), fusarinine C (Fusarium roseum), yersiniabactin (Yersinia pestis), vibriobactin (Vibrio cholerae), azotobactin (Azotobacter vinelandii), pseudobactin (Pseudomonas B 10), erythrobactin (Saccharopolyspora erythraea) or ornibactin (Burkholderia cepacia).

    Pseudomonas Siderophores

    Like all aerobic bacteria, pseudomonads need to take up iron via the secretion of siderophores which complex iron (III) with high affinity. Much progress has been made in the elucidation of siderophore-mediated high-affinity iron uptake by Pseudomonas, especially in the case of the opportunistic pathogen, P. aeruginosa. Fluorescent pseudomonads produce the high-affinity peptidic siderophore pyoverdine, but also, in many cases, a second siderophore of lesser affinity for iron. Some of the genes for the biosynthesis and uptake of these siderophores have been identified and the functions of the encoded proteins known. Iron uptake via siderophores is regulated at several levels, via the general iron-sensitive repressor Fur (Ferric Uptake Regulator), via extracytoplasmic sigma factors/anti-sigma factors or via other regulators. Since pseudomonads are ubiquitous microorganisms, it is not surprising to find in their genome a large number of genes encoding receptors for the uptake of heterologous ferrisiderophores or heme reflecting their great adaptability to diverse iron sources. Another exciting development is the recent evidence for a cross-talk between the iron regulon and other regulatory networks, including the diffusible signal molecule-mediated quorum sensing in P. aeruginosa.