It is assumed that reactive oxygen species (ROS) play a key role in inflammatory bowel diseases and colon cancer and a number of studies indicate that lactic acid bacteria (LAB) possess antioxidant properties and may prevent these diseases. In the present study, we developed a model which allowed us to investigate the prevention of oxidative DNA damage in human derived colon (HT29) cells by LAB. Furthermore, we investigated if these effects correlate with superoxide (O2(-)) resistance of the strains. The protective properties of 55 strains were monitored in single cell gel electrophoresis (SCGE) assays. After preincubation of the cells with LAB (60 min), oxidative damage was induced by exposure to plumbagin (5.0 microM, 120 min) which releases O2(-) or by hydrogen peroxide (50 microM, 10 min); O2(-) resistance was monitored in plate growth inhibition assays. 25 strains (45%) reduced plumbagin induced DNA migration while only few of them (20%) were protective towards hydrogen peroxide induced damage. The strongest effects (up to 60% reduction of O2(-) induced DNA migration) were observed with representatives of the species Streptococcus thermophilus. The prevention of DNA damage in the colon cells by the bacteria did not correlate with their O2(-) resistance. Additional experiments indicate that the reduction of oxidative damage is only seen with viable bacteria but not with heat inactivated cells and that it takes also place when the colon cells are separated from the LAB by permeable filter membranes indicating that the bacteria release ROS protective factors into the medium. Dose response experiments showed that the protection depends on the concentration of the bacteria; significant effects were observed with titers 3 x 10(6-7)cells/ml. Unexpectedly, we found that a substantial fraction of the strains (13%) induced DNA damage in untreated cells, some of them increased also the effects of the ROS generating chemicals. Preliminary experiments with tetramethylbenzidine (TMB) agar indicate that this phenomenon may be due to release of hydrogen peroxide by the bacteria. Overall, our study shows that the impact of LAB on DNA damage in human derived colon cells is ambivalent; while the majority of strains was protective against oxidative damage some of them induced per se pronounced DNA migration. Since the effects were seen with bacterial concentrations which may be reached in the intestinal tract after consumption of fermented milk products, it is likely that the effects we observed in the present study are relevant for humans.