Summary: The capacity of culture filtrates of growing in iron-deficient medium to solubilize inorganic iron increases throughout the growth phase. When the source of iron, either ferric phosphate or ferritin, was contained within diffusion capsules, bacteria grew unimpeded indicating that freely-diffusible iron chelates were being produced which could support growth. The maximum production of the iron-binding compounds, termed exochelins, occurred with 0·03 μg iron ml in the medium; with 1·0 μg iron ml (giving iron-sufficient growth) the amount of exochelin produced was less than 0·1% of the maximum level. Using ion-exchange chromatography at least six exochelins were recovered from culture filtrates. The major compound, constituting about 60% of the total, was isolated, purified and partially characterized. It solubilized iron from inorganic and organic forms, stimulated growth of in serum, was freely diffusible in both its ferri and desferri forms, and transferred the chelated iron to cells at a rate which was independent of its concentration.


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