Iron transport in Salmonella typhimurium: mutants blocked in the biosynthesis of enterobactin

Abstract

A number of mutants of Salmonella typhimurium were isolated which are blocked in the biosynthesis of enterobactin, an iron chelator that is secreted by the wild-type bacteria when they are grown on low iron media. One class of these enb mutants accumulates the enterobactin precursor 2,3-dihydroxybenzoic acid, and another class does not accumulate any detectable catechol precursor. The enb mutants grow very well on a glucose-mineral salts medium. Addition of citrate, itself an iron chelator, to the medium drastically inhibits growth unless the medium is supplemented with enterobactin or iron salts. Citrate inhibits iron uptake from the medium by enb mutants; enterobactin counteracts this inhibition and also, by itself, increases iron uptake. Thus, the apparent function of enterobactin is to promote the absorption of iron from the medium by the bacteria. Transduction experiments showed that the genes for enterobactin biosynthesis are closely linked on the S. typhimurium chromosome. It is suggested that they form an operon which is repressed by the presence of iron. S. typhimurium can utilize the iron chelate ferrichrome. (Deferriferrichrome is a cyclic hexapeptide that is produced by some fungi but not by S. typhimurium.) The enb mutants use ferrichrome as an effective growth factor.