The critical role of iron in host-bacterial interactions

The ability of potential pathogens to acquire iron in a host is an important determinant of both their virulence and the nature of the infection produced. Virulent gram-negative bacteria are capable of acquiring sufficient iron from the host because their virulence (for chick embryos) is unaffected by exogenous iron. Avirulent mutants which are apparently limited in their ability to acquire iron could be isolated from the virulent strains. The lethality of these mutants was significantly enhanced by exogenous iron. Reduction of the relatively high serum iron saturation of chick embryos (to levels more closely approximating those in man) by pretreatment with iron-binding proteins or endotoxin inhibits the lethality of some virulent bacteria. Those bacteria whose virulence was reduced include the Shigella, Vibrio cholerae and strains of Neisseria gonorrhoeae, all of which are nondisseminating pathogens in the normal human host. Pathogens which produce septicemic and disseminating infections such as Neisseria meningitidis, Haemophilus influenzae type B, Escherichia coli possessing K-1 antigen, Pseudomonas aeruginosa and Salmonella typhimurium and disseminating strains of N. gonorrhoeae were, in general, unaffected by reduced serum iron saturation. These disseminating bacteria appeared to produce greater quantities of compounds (siderophores) which stimulated microbial growth in low-iron media than did the nondisseminating pathogens. Thus, the gram-negative bacteria tested can be divided into four major classes according to their responses to modifications in iron levels in the chick embryo model and these results correlate with the nature of the infections which they typically produce in man.