Bacteria and fungi also synthesize iron transfer compounds, called siderochromes.The common structures are complex hydroxamates (also called ferrichromes or ferrioxamines) or complex catechols (enterobactin). They have peptide backbones and are very strong chelating agents (Kf ~ 1030 to 1050), allowing the organism to extract iron from surroundings that contain very little iron or are basic enough that iron is present as insoluble hydroxides or oxides. Some of these compounds act as growth factors for bacteria and others act as antibiotics. There are also examples in which the iron is bound by a mixture of phenolic hydroxyl, hydroxamate, amine and alcoholic hydroxyl groups. Siderochromes are substances which are excreted into the culture medium by micro- organisms in iron-deficient conditions. These compounds characteristically bind Fe3 + very strongly, and a number of them participate in the transport of iron into the cells. Siderochromes are synthesized in large amounts and excreted into the culture medium only when the bacteria have insufficient iron, and it forms a very stable complex with Fe(II).
Gary L. Miessler, Donald A. Tarr, Inorganic Chemistry, Third Edition, 2004.