Superoxide anions are required for the inactivation ofEscherichia coli induced by high concentrations of hydrogen peroxide

The wild-type strain and mutants ofEscherichia coli lacking Mn-superoxide dismutase (Sod A) or Fe-superoxide dismutase (Sod B) are compared for their sensitivity to the H₂O₂ insult (exposure for 15 min at 37°C, in M9 salts). Whereas mode one killing is similar in superoxide dismutase mutants and wild-type cells, the latter strain appears to be more resistant than the former ones to mode two lethality. Furthermore, Sod B cells, as well as wild-type cells but unlike Sod A cells, are capable of reversing the toxicity of the oxidant (even in the presence of chloramphenicol), this effect being observed by gradually increasing the H₂O₂ concentration from 2.5 to 10 mM. It is concluded that (a) superoxide ions may not be involved in the production of mode one killing by H₂O₂, although further experiments are needed to validate or modify this hypothesis; (b) superoxide ions mediate mode two killing by H₂O₂, possibly by reducing trivalent iron to the divalent form; and (c) the intervening zone of partial resistance observed in wild-type and Sod B cells exposed to intermediate H₂O₂ concentrations is not a consequence of Mn-superoxide dismutase induction; it would appear, however, that cells lacking this superoxide dismutase isoenzyme are not proficient in this acquired response.