With the possible exception of certain strains of lactic acid bacteria, iron is known to be an essential element for all species of microbes and for all higher organisms. Within the cell, iron protein catalysts participate in some of the reactions most fundamental to life, including aerobic and anaerobic energy metabolism, nitrogen fixation, photosynthesis, and generation of the deoxyribotides required for synthesis of DNA. Iron is abundant in the environment but may be insoluble or in some other way unavailable. Mechanisms for assimilation of iron have been evolved by all species but, because of the plasticity and diversity of microbial metabolism, those in unicellular organisms are the most readily studied. The particular system of microbial iron acquisition investigated in this laboratory is labelled high-affinity and consists of low molecular weight, virtually Fe(III) specific ligands, termed siderophores, and the matching membrane receptors for the complexed Fe(III) ion. This system, which is coordinately expressed under iron starvation, has been detected in virtually all aerobic and facultative anaerobic microorganisms carefully examined for its presence [1]. In general, siderophores can be relegated to one of two chemical classes, viz., hydrozamic acids and catechols. The number of siderophores characterized to date, some by crystallography and/or high resolution nuclear magnetic resonance spectroscopy, must now number several score. The complete systems, comprised of siderophore and specific receptor, is the converse is true, possibly because of the aromatic character of enterobactin and its propensity to adhere to proteins such as albumin.