Although sulfonamide residues are frequently reported as freshwaters contaminants, information on the ability of native bacteria to modify these synthetic antibiotics is scarce. Our purpose was to investigate the potential of bacteria from different aquatic environments to cleave or transform sulfamethoxazole (SMX) and infer on their ability to reduce the toxicity of this antibiotic. From a collection of about 100 Proteobacteria, 47 strains previously isolated from drinking water, surface water, and wastewater grew in the presence of 200 mu M-SMX, and were further studied. Out of these, 14 strains, mostly from mineral drinking water, transformed SMX into equimolar amounts of the lesser toxic derivative N-4-acetyl-sulfamethoxazole. The highest percentage of SMX transformation was recorded for two strains affiliated to Pseudomonas mandelii. For P. mandelii McBPA4 higher SMX transformation rate and extent were observed in fed-batch (similar to 8 mu M-SMX/h, 81%) than in batch conditions (similar to 5 mu M-SMX/h, 25%), but similar specific transformation rates were found in both cultivation modes (similar to 20 mu mol(SMX)/g(cell dry weight)/h), indicating the dependence of the process on the microbial load. These results evidence that the capacity to transform synthetic antibiotics may be common among bacteria and highlight the potential of environmental bacteria in attenuating the potential adverse effects of pollution with sulfonamides.