Bacterial iron enhances oxygen radical-mediated killing of Staphylococcus aureus by phagocytes

It has been shown that increasing bacterial iron concentration enhances killing by hydrogen peroxide (H2O2) but not by polymorphonuclear granulocytes (PMN). It is possible that owing to the multiple bactericidal mechanisms of the PMN, differences in the killing rate of iron-loaded bacteria and control bacteria are obscured. We decided, therefore, to compare the killing of iron-loaded bacteria with that of control bacteria using human monocytes (MN), PMN, and PMN-derived cytoplasts. Incubation of Staphylococcus aureus with increasing concentrations of ferrous ammonium sulfate (0 to 1,000 microM) progressively increased the iron content in the bacteria (from 0.01 to 0.24 mumol of iron per 10(9) bacteria). Iron loading of the bacteria markedly increased their susceptibility to killing by H2O2. After 1 h of incubation with 1 mM H2O2, 95 +/- 2% of the iron-loaded bacteria were killed compared with 18 +/- 4% of the control bacteria (P less than 0.0001). Iron loading of bacteria did not alter their susceptibility to killing by human PMN. However, iron-loaded bacteria were more susceptible to killing by MN (after 12 min of incubation, 81 +/- 2 versus 74 +/- 2% killing; P less than 0.008) and to killing by PMN-derived cytoplasts (after 60 min of incubation, 52 +/- 8 versus 33 +/- 5%; P = 0.003) than the controls. Moreover, iron loading enhanced luminol-mediated chemiluminescence of MN, PMN, and PMN-derived cytoplasts. The hydroxyl radical scavenger thiourea inhibited H2O2-mediated killing of iron-loaded staphylococci as well as luminol-mediated chemiluminescence. These results suggest that alterations in intrinsic iron content increase killing of staphylococci by H2O2, MN, and PMN-derived cytoplasts by a free radical-mediated mechanism.