We investigated the mechanism of the oxidative DNA damage induction by exposure to O-2 in Prevotella melaninogenica, a strict anaerobe. Flow cytometry with hydroethidine and dichlorofluorescein diacetate showed that O-2 exposure generated O-2(.-) and H2O2. Results of electron spin resonance with alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone and ethanol showed that O-2 exposure also induced (OH)-O-. radical generation in P. melaninogenica loaded with FeCl2 but not in samples without FeCl2 loading. In P. melaninogenica, O-2 exposure increased 8-hydroxydeoxyguanosine (8OHdG), typical of oxidative DNA damage. Catalase inhibited the increase, but the (OH)-O-. radical scavengers did not. Phenanthroline, a membrane-permeable Fe and Cu chelator, increased the 8OHdG induction. In FeCl2-loaded samples, induction of 8OHdG decreased. Addition of H2O2 markedly increased 8OHdG levels. These results indicate that in P. melaninogenica, exposure to O-2 generated and accumulated O-2(.-) and H2O2, and that a crypto-OH radical generated through H2O2 was the active species in the 8OHdG induction. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.