Iron transport in Mycobacterium smegmatis: the location of mycobactin by electron microscopy

Mycobactin, the lipid-soluble iron-binding compound of the mycobacteria, has been located using electron microscopy of whole cells stained with vanadate. It forms a discrete, discontinuous region close to, or even included in, the cytoplasmic membrane of Mycobacterium smegmatis. It does not occur throughout the whole thickness of the wall but is some distance within the envelope. Possible models for the accommodation of mycobactin are discussed. It is concluded from these observations that mycobactin may be acting either as a store of iron, or as an ionophore, or possibly fulfilling both roles.