Siderophores: Biochemical Ecology and Mechanism of Iron Transport in Enterobacteria

Siderophores protect Escherichia coli and Salmonella typhimurium from certain phages, bacteriocins, and antibiotics by two mechanisms. The first is adsorption competition for outer membrane receptors. Thus ferrichrome competes with T1, T5, Φ80, colicin M, and albomycin for a common site (tonA) in Escherichia coli and with phage ES18 and albomycin in Salmonella. Ferric enterobactin similarly antagonizes colicin B. In the second mechanism siderophores nonspecifically protect against the B group colicins in an event requiring use of siderophore iron. Ferric enterobactin and cognate membrane receptors are overproduced at low levels of iron. Experiments with 55 Fe and tritiated ligand and with the isostructural chromic analog show that ferrichrome rapidly delivers its iron while the ligand more slowly, although again as the iron complex, penetrates the cell.