Abstract
Certain strains of Escherichia coli associated with bacteremia of humans and domestic animals harbor plasmids that promote efficient iron uptake. The mechanism, which is an important component of the virulence of invasive strains, is independent of the enterobactin system for iron uptake. Plasmid-specified siderophore was assayed by its ability to support the growth of a chelator-deficient mutant in conditions of iron deprivation. The chelator, which was chemically determined to be a hydroxamate compound, was identical on the basis of field desorption mass spectrometry with aerobactin, a siderophore synthesized by Aerobacter aerogenes. In conditions of iron stress, aerobactin is secreted into the culture medium of plasmid-bearing E. coli strains. Reconstruction experiments involving a chelator-deficient mutant growing with exogenous chelator suggested that association of a small fraction of the total siderophore synthesized with cellular material is due to transient binding of aerobactin to membrane receptors during active bacterial growth.