Unlike superoxide anion, which is short-lived and local in its effect, hydrogen peroxide is longer-lasting and membrane-permeable, so it can diffuse away from the site of production (Winterbourn CC et al. 2006). Though H2O2 can permeate bacteria, it is unlikely to be directly bactericidal at the concentrations achieved in the phagosome. Its relatively benign nature is explicable in terms of its chemistry. Although it has a high two-electron reduction potential (H2O2/H2O;1.77V) and is therefore a strong oxidant, a high activation energy makes it a kinetically sluggish oxidant of most biomolecules (Winterbourn CC et al. 2006). However, the derivatives of H2O2 such as hydroxyl radical (OH.) are far more reactive (Root RK & Metcalf JA 1977; Winterbourn CC et al. 2006). Hydroxyl radical is produced by interaction of Fe2+ with hydrogen peroxide (Fenton reaction). Rates of reaction with iron-sulfur clusters are sufficiently fast for H2O2 to damage dehydratases and kill bacteria by mechanisms in which site-directed Fenton chemistry targets vulnerable molecules in the bacterial cytosol and the bacterial DNA (Keyer K & Imlay JA 1996; Jang S & Imlay JA 2007).