Scheme illustrating the effects of reactive oxygen species (ROS) into the cell (a) and antioxidant defense mechanisms (b) evaluated in this work. (a) Growth in the presence of reactive oxygen species (ROS): superoxide radical [O2.-], hydrogen peroxide [H2O2], hydroxyl radical [OH-] and hydroperoxyl radical [HOO-]. These ROS can damage nucleic acids (RNA and DNA) as well as proteins and lipids, leading to cell death. (b) The superoxide dismutases (SOD), which are cytosolic (Mn-SOD and Fe-SOD) and periplasmic (Cu-SOD) allow O2.- detoxification. The catalase activity responsible for the reduction of H2O2 to H2O is effected by two hydroperoxydases [hydroperoxydase I (HPI) and hydroperoxydase II (HPII)], and the alkyl hydroperoxydase (AhpC). The glutathione is synthetised from glutamate, cysteine and glycine, by 2 unrelated ligases: the γ-glutamylcysteine synthetase (GshA) and the glutathione synthetase (GshB). The glutathione oxidoreductase (Gor) reduces glutathione disulfide (GSSG), which is formed upon oxidation. The glucose 6-phosphate dehydrogenase (G6PDH) allows indirectly the reduction of NADP to NADPH. The Fenton reaction is the result of electron transfer from donor to H2O2 catalyzed by iron Fe3+. The stars show dosages effected in this study.