The fate of virulent and avirulent strains of Salmonella typhimurium in untreated and iron-injected mice and in transferrin-containing media demonstrated a direct relationship between bacterial virulence and the ability of bacteria to acquire transferrin-bound iron. Effects of injected iron on the development of infections with virulent and avirulent bacterial strains were determined in normal and immune mice by determinations of bacterial numbers in tissue homogenates and the mortality of infected animals. Results showed that infected and ironinjected mice died much more rapidly and frequently from overwhelming infections than infected and saline-injected mice. The infection-promoting effect of iron varied with the degree of bacterial virulence; the more virulent the bacteria, the more helpful was iron for the development of lethal infections. Siderophores promoted lethal infections in mice infected with virulent but not with avirulent bacteria. Experiments with vaccinated animals showed that iron exerted a deleterious effect on acquired immunity. Immune mice infected with virulent bacteria and injected with iron developed lethal infections as rapidly and nearly as frequently as similarly treated normal mice. Siderophores did not promote the development of lethal infections in immune mice. The effectiveness of iron, but not of siderophores, to promote bacterial infections in vaccinated mice revealed that acquired immunity is dependent upon the activity of an iron-neutralizable antibacterial system.

The effect of iron on resistance to Salmonella typhimurium was investigated in mice inoculated with vaccines prepared from live and avirulent (SL3770) or killed and virulent (SR11 or LT2) bacteria. It has been found that mice vaccinated with SL3770 vaccine develop an immunity which can be neutralized with iron. Iron promoted the development of lethal infections by serving as a growth-essential nutrilite for infecting bacteria and by neutralizing the acquired immunity. The titration of this dual effect of iron showed that more iron was needed to neutralize the immunity in vaccinated animals than to promote bacterial growth in normal animals. In the presence of a sufficient amount of exogenous iron, as few as 10 bacteria caused lethal infections in normal and immune mice with the same effectiveness. This iron-sensitive immunity could be changed to iron-resistant immunity by the immunological stimulation of SL3770-vaccinated mice with a sonicated vaccine prepared from heat-killed SR11 or LT2 bacteria. In distinction to iron-sensitive immunity, iron-resistant immunity could be transferred from SR11or LT2-stimulated to normal mice with serum. Although effective in the transfer of antibacterial immunity, sera of SR11or LT2-stimulated mice supported the growth of virulent bacteria as well as did sera of normal mice. The absorption of immune serum with either SR11 or LT2 bacteria removed its protective quality, but the sensitized bacteria remained as infectious as untreated bacteria for iron-treated normal mice. Only in SL3770-vaccinated mice were the immune serum-sensitized bacteria not able to cause the infection in spite of daily treatment with iron. These results suggest that iron-resistant immunity is due to the synergistic action of specific antibody and phagocytes of immunologically stimulated animals.