Iron is an essential cofactor for almost all living organisms, including bacteria. However, the solubility of iron is very low under aerobic conditions at neutral pH, and in many environments bioavailable iron is not sufficient to sustain bacterial growth. Microbial pathogens encounter an iron-restricted environment also in the host where iron is bound to proteins (hemoglobin, myoglobin, cytochromes) or to high-affinity iron-carriers (transferrin, lactoferrin, ferritin). To acquire iron under these conditions, bacteria have evolved a number of different strategies, the most common is siderophore-mediated iron uptake. Siderophores are high-affinity iron chelators which are released to the extracellular environment where they complex iron and deliver it to the bacterial cell, via specific outer membrane receptors. Our lab is interested in pyochelin, a siderophore made by the opportunistic human pathogen Pseudomonas aeruginosa. Pyochelin biosynthesis occurs by a thiotemplate mechanism from salicylate and two molecules of cysteine. We have cloned the genes involved and have contributed to the elucidation of the biosynthetic pathway. In P. aeruginosa, the first cysteine residue is converted to its D-isoform during thiazoline ring formation whereas the second cysteine remains in its L-configuration, thus determining the stereochemistry of the two interconvertible pyochelin diastereoisomers as 4'R, 2"R, 4"R(pyochelin I) and 4'R, 2"S, 4"R (pyochelin II). We recently found that the Pseudomonas fluorescens strains Pf-5 and CHA0 make a different stereoisomeric mixture and we determined its structure to be enantio-pyochelin, the optical antipode of the P. aeruginosasiderophore. The biosynthesis of enantio-pyochelin is currently being investigated in our laboratory.