Expression of hydroxamate and phenolate siderophores by Shigella flexneri

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Abstract
S. flexneri strains were assayed for the ability to synthesize and utilize phenolate and hydroxamate siderophores. The hydroxamate aerobactin was synthesized by all isolates tested, whereas phenolates were only rarely produced. Expression of aerobactin was accompanied by production of a single Fe-regulated outer membrane protein (MW = 74,00). This protein was not produced by a mutant defective in aerobactin utilization and may serve as the aerobactin receptor. Phenolate (enterobactin)-producing strains synthesized 3 additional outer membrane proteins (MW = 74,000, 81,000 and 83,000) in response to Fe starvation. These proteins are the same apparent size as those produced by Escherichia coli K-12 strains. Ent sequences are apparently present in strains which do not synthesize this compound. Although normally silent, ent genes can be activated in Ent- strains to produce Ent+ variants. These laboratory variants are phenotypically indistinguishable from clinical Ent+ isolates.