Bacteroides thetaiotaomicron, a bowel anaerobic commensal, seems to release enzymes detoxifying reactive oxygen species according to our recent work. This opportunistic pathogen would be beneficial in the case of an inflammatory process. To explore its role after an oxidative or nutritive stress, six to seven separate experiments were performed. The bacteria were grown on media restricted in growth factors or supplemented with bile. Their viability was checked after surface protein extraction. The extracts underwent 2D electrophoresis. Gel images were statistically analysed to construct "master" gels. Proteins were identified (peptide-mass fingerprinting technique). The effect of each extract on superoxide anions was evaluated (spectrophotometric method). Superoxide dismutase was identified and a major superoxide anion inhibition was shown by extracts obtained after a nutritive and oxidative stress without significant bacterial death. So, a therapeutic antioxidant potential is firmly hoped for. [figure: see text]