Effect of iron compounds on antibacterial function of human polymorphs and plasma

Human plasma was bactericidal for small numbers of Klebsiella pneumoniae, but larger numbers grew slowly over a period of 24 h. Human polymorphs in a clot of autologous plasma had a bacteriostatic effect on relatively large numbers of bacteria for up to 24 h and were much more effective than plasma alone. The bactericidal effect of plasma could be abolished by saturating the plasma albumin and hemopexin with hematin, the haptoglobin with hemoglobin, and the transferrin with Fe3+. Stimulation of bacterial growth in the presence of polymorphs depended on the degree of saturation of the plasma proteins which bind the Fe3+ or heme compounds. Hematin bound to albumin appeared to be readily available to K. pneumoniae. Free hemoglobin stimulated bacterial growth but the organism could not utilize the haptoglobin-hemoglobin complex. In the presence of polymorphs the addition of Fe3+ to give 60% saturation of the transferrin with iron led to rapid growth after a long delay (19 h). Progressive increases in saturation above 60% gave correspondingly quicker growth, suggesting that the higher the saturation of transferrin, the easier it is for the bacteria to acquire iron. The antibacterial effect of the plasma appears to be critically dependent on the availability of iron for the bacteria, and this in turn affects the ability of the polymorphs to control bacterial growth.