Extracellular iron acquisition by mycobacteria: role of the exochelins and evidence against the participation of mycobactin.

Mycobacterium bovis var. BCG was grown under iron-deficient conditions in the presence and absence of 1% Tween 80. Mycobactin, the iron iron ionophore of mycobacteria, was found solely within the bacteria grown in the absence of Tween, but low concentrations (0.75 mug/ml) of it appeared in the medium in the presence of the surfactant. Both types of medium contain agents, named exochelins, which could solubilize iron. 55Fe added to spent culture media was recovered only chelated to these compounds. Two exochelins were detected, isolated, and purified. Neither were precursors or breakdown products of mycobactin. In the desferri-form, exochelin MB-2, the major component, reversed the inhibitory effect of serum on the growth of BCG, and in their ferri-forms exochelins MB-1, MB-2, and MS (from Mycobacterium smegmatis) stimulated the growth of their producing organism in the presence of serum. Exochelin MB-2 could physically remove iron from ferritin, and BCG used ferritin as a source of iron during growth even when ferritin was separated from the bacteria by a dialysis membrane. As solutions of the exochelins were freely dialyzable, whereas solutions of mycobactin, even in Tween, were not, only exochelin could have been active in this experiment. The exochelins are proposed as the functional extracellular iron-binding agents of BCG and other mycobacteria, the role of mycobactin being confined to that of a cell wall iron transporter.