Siderophore

Siderophores (Greek: "iron carrier") are small, high-affinity iron-chelating compounds secreted by microorganisms such as bacteria, fungi and grasses. Siderophores are amongst the strongest soluble Fe³⁺ binding agents known.

Iron is essential for almost all life for processes such as respiration and DNA synthesis. Despite being one of the most abundant elements in the Earth’s crust, the bioavailability of iron in many environments such as the soil or sea is limited by the very low solubility of the Fe³⁺ ion. This is the predominant state of iron in aqueous, non-acidic, oxygenated environments. It accumulates in common mineral phases such as iron oxides and hydroxides (the minerals that are responsible for red and yellow soil colours) hence cannot be readily used by organisms. Microbes release siderophores to scavenge iron from these mineral phases by formation of soluble Fe³⁺complexes that can be taken up by active transport mechanisms. Many siderophores are nonribosomal peptides, although several are biosynthesised independently.

Siderophores are also important for some pathogenic bacteria for their acquisition of iron. In mammalian hosts, iron is tightly bound to proteins such as hemoglobin, transferrin, lactoferrin and ferritin. The strict homeostasis of iron leads to a free concentration of about 10⁻²⁴ mol L⁻¹, hence there are great evolutionary pressures put on pathogenic bacteria to obtain this metal. For example, the anthrax pathogen Bacillus anthracis releases two siderophores, bacillibactin and petrobactin, to scavenge ferric iron from iron proteins. While bacillibactin has been shown to bind to the immune system protein siderocalin, petrobactin is assumed to evade the immune system and has been shown to be important for virulence in mice.

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