Effects of iron on the growth of avirulent and virulent strains of Escherichia coli were tested in mice and in mammalian sera. Infection of the animals with iron increased mortality rates in mice infected with the avirulent strain to levels found in mice infected with the virulent strain. In vitro experiments showed that bacteria deprived of iron in bovine or human sera or milk or in chicken egg white stopped miltiplication and died in a very short time. These antibacterial effects were neutralized effectively with the addition of exogenous iron or the iron-binding bacterial product, enterochelin. In contrast to avirulent bacteria, which were effectively inhibited in mammalian serum, virulent bacteria were able to obtain iron and multiply. The ability of virulent bacteria to grow in mammalian serum is being attributed to the presence of iron-binding enterochelin and lipopolysaccharide in large amounts on the cell walls of virulent bacteria.

Two smooth and six rough strains of Salmonella typhimurium with progressively smaller amounts of sugar and protein in their outer membrane were tested for degree of virulence in normal and iron-injected mice and for ability to acquire iron in mammalian sera. The rate of mortality showed that bacterial virulence for mice was lowered with progressive decrease of outer-membrane sugar and protein. Iron injections increased the rate of mortality in mice infected either with smooth strains or with superficially rough strains but were without effect in mice infected with deep rough strains. In in vitro experiments, iron promoted with equal effectiveness the growth of all serum-exposed bacterial strains, whereas enterobactin (E) was much more effective in promoting the growth of smooth and superficial rough than in promoting that of deep rough strains. Various experiments showed that deep rough strains cannot grow in E-supplemented serum because they are not able to use the transferrin-iron-E complexes that E forms with transferrin-iron. This failure to use transferrin-iron-E complexes by deep rough strains was found to be due to the inability of these strains to adsorb iron-containing complexes to their outer membrane. Adsorption studies with chemically treated bacteria showed that the receptor of transferrin-iron-E or Eiron complexes is a protein of the outer membrane of bacterial cells.