The effects of different dissolved oxygen (DO) concentration (as % air saturation) on the oxidative stress response of Bacillus sp. F26 to menadione were investigated in a fermenter. The culture response to the oxidative stress produced by continuous addition of menadione (50 mu M/h) was characterized in terms of the intracellular activities of two key defensive enzymes, superoxide dismutase (SOD) and catalase (CAT), cell growth and lipid peroxidation. It was found that oxidative damage to exponential cells induced by menadione was amplified by incremental DO concentration, compared with the untreated cells, which was indicative of the decrease in biomass (DCW) and the greater increase in malonedialdehyde (MDA) concentration when DO concentrations were raised from 25 to 55% and 80% saturated DO. As a result, increments of SOD and CAT activities during the exponential phase of growth were well associated with the enhancement of DO concentrations, whereas no inhibition of growth and induction of enzymes were observed in comparison to untreated cells below 5% saturated DO. The present results show that the formation of reactive oxygen species (ROS) induced by menadione is oxygen-dependent. Higher DO concentrations outside the cells can amplify the oxidative stress response of Bacillus sp. F26 to menadione. (c) 2005 Elsevier Ltd. All rights reserved.