Although iron is one of the most abundant elements on the planet, bacteria that require iron as an essential nutrient face an enormous challenge in obtaining enough of it in a bioavailable form. In soil, for example, the amount of free ferric iron is vanishingly small, and in a mammalian host most iron is already bound to proteins such as transferrin, hemoglobin, and ferritin.
In order to get at these precious iron sources, bacteria secrete molecules known as siderophores (the word comes from the Greek: sideros = “iron", phoros = "-bearing”).
Siderophores bind ferric iron with incredibly high affinity – high enough to soak up scarce ferric iron from the soil, or to strip an iron atom that’s already bound to a mammalian host protein. Once a siderophore has bound iron, bacteria employ a network of proteins to take up the iron-siderophore complex such that the iron can be eventually used in metabolic processes.