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Aerobic microorganisms have evolved a variety of siderochromes, special ligands which can dissolve insoluble ferric iron and facilitate its transport into the cell. We have found that enb mutants of Salmonella typhimurium blocked in the biosynthesis of enterobactin (its natural iron carrier) are able to utilize siderochromes of different types made by other microorganisms as iron carriers. The antibiotic albomycin 62 was used to select mutants defective in ferrichrome-mediated iron uptake. Twelve classes of albomycin-resistant mutants, named sid, were defined on the basis of their growth responses to other siderochromes. Most of these classes have genetic lesions in loci that are cotransduced with panC (represented at 9 min on the genetic map). The locus designated sid.J is cotransduced with enb, whereas sidK and sidL are linked with neither panC nor enb. Genetic and physiological data indicate that S. typhimurium has several transport systems of high specificity for a variety of siderochromes produced by other microorganisms.

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 ofcitrate, itselfan ironchelator, tothe 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. In recent years, iron-sequestering agents consisting of 2, 3-dihydroxybenzoic acid (DHBA) conjugated with the amino group(s) of a specific amino acid have been isolated from several bacteria: 2, 3-dihydroxybenzoyl (DHB)-glycine from Bacillus subtilis (4); DHB-serine from Escherichia coli (1, 7), Aerobacter aerogenes (7), and Salmonella typhimurium (16); and di-DHBlysine from Azotobacter vinelandii (3). S. typhimurium also produces enterobactin (10), a cyclic trimer of DHB-serine. These compounds are all

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Salmonellosis pacifarin activity is detected by an increased survivorship of mice, doubly infected with avirulent and virulent Salmonella typhimurium, when heretofore unknown agents, found in certain natural foodstuffs and in the supernatants of certain bacterial cultures, are fed to the infected animals as dietary supplements. We now announce the identity of one of these agents: it is enterobactin, a cyclic trimer of 2,3-dihydroxy-N-benzoyl-L-serine. Basal diets enriched with as little as 2 mg of crystalline trimer per kg of diet show pacifarin activity to a statistically significant degree. Diets supplemented with as much as 100 mg of the monomer per kg of diet show no such activity.