Bacterial solutions to the iron-supply problem.

The insolubility of Fe3+ necessitates special mechanisms for iron acquisition in most organisms. Bacteria use siderophores to chelate Fe3+ and iron in heme, hemoglobin, transferrin and lactoferrin, and employ novel mechanisms for receptor-dependent iron transport and iron-regulated gene expression. These mechanisms involve transfer of energy from the cytoplasmic membrane to the outer membrane to drive active transport and might induce transcription of transport genes by transmitting a signal from the cell surface.