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Abstract

Iron is an essential element for many key redox systems. It is difficult to acquire for cells under oxic conditions, since Fe3+ forms insoluble hydroxides. In the human host, iron is tightly bound to proteins. Bacteria invented iron transport systems which solubilize external Fe3+ by secreted low-molecular weight compounds, designated siderophores, or directly from the human proteins. Gram-negative bacteria contain an intricate energy-coupled iron transport mechanism across the outer membrane which lacks an energy source. The electrochemical potential of the cytoplasmic membrane delivers the energy. Transport across the cytoplasmic membrane is most frequently achieved by ABC transporters in Gram-positive and Gram-negative bacteria. Under anaerobic conditions, iron is the soluble Fe2+ form and transported different to Fe3+. Iron transport and intracellular iron concentrations are controlled by transcription regulation of iron transport genes. Transcription is turned on under iron-limiting growth conditions which usually exist in natural environments.