The hydrogen peroxide dose-response curves forEscherichia colikilling and DNA nickingin vitrodisplay remarkably similar bimodal patterns. The concentrations of the oxidant resulting in maximum mode one killing, however, exceeds by two orders of magnitude those resulting in the mode one DNA nicking response. Addition of histidine differentially affects the experimental curves describing the dose-dependency for bacterial killing and DNA damagein vitro. Indeed, the lethal effect elicited by the oxidant in the presence of the amino acid is strictly concentration-dependent and thus the inactivation curve loses its bimodal character. In marked contrast, histidine abolishes DNA damage generated by low concentrations of hydrogen peroxide (<100μM) in thein vitrosystem (the mode one DNA nicking response) but greatly increases DNA damage produced by concentrations of the oxidant higher than 1 mM (the mode two DNA nicking response).Experimental results also suggest that treatment of covalently closed circular double-stranded super-coiled DNA with hydrogen peroxide, in the presence of both histidine and iron, may result in the formation of DNA double strand breakage, a type of lesion which is not efficiently produced by the oxidant in the absence of the amino acid.Taken together, the above results indicate that histidine differentially affects thein vitroDNA cleavage andE. colilethality induced by hydrogen peroxide and suggest that different molecular events mediate mode one DNA nickingin vitroand mode one killing of bacterial cells.