Iron Supply as a Virulence Factor

Summary

Iron, an essential nutrient for bacteria, is withheld by the host by binding to transferrin of the serum and to lactoferrin in secretory fluids. Bacteria have developed elaborate iron supply systems to overcome iron deprivation. Usually, they synthesize and secrete compounds of low molecular weight, termed siderophores, that complex Fe³⁺ with extremely high affinity and specificity. The iron-siderophore complexes are taken up by specific transport systems. In Escherichia coli, at least five different transport systems exist, controlled by about 30 genes. They effectively compete with the iron limitation devices of the host.

Synthesis of a number of exotoxins is induced by iron limitation, including a cell-bound hemolysin of various Serratia strains. Lysis occurs within 30 min after contact of living Serratia cells with human erythrocytes (cell ratio 1:1). Surprisingly, erythrocyte membrane proteins are hydrolyzed at a few sites. The hemolysin resides in the membrane fraction of Serratia and can be inactivated by incubating cells with proteases under conditions such that they remain viable, suggesting that the hemolysin is exposed at the cell surface.