Bacteria used for the production of yogurt inactivate carcinogens and prevent DNA damage in the colon of rats

Lactic acid-producing bacteria prevent carcinogen-induced preneoplastic les ions and tumors in rat colon. Because the mechanisms responsible for these protective effects are unknown, two strains of lactic acid bacteria, Lactob acillus delbrueckii ssp. bulgaricus 191R and Streptococcus salivarius ssp. thermophilus CH3, that are used to produce yogurt, were investigated in vit ro and in vivo to elucidate their potential to deactivate carcinogens. Usin g the "Comet assay" to detect genetic damage, we found that L. bulgaricus 1 91R applied orally to rats could prevent 1,2-dimethylhydrazine-induced DNA breaks in the colon in vivo, whereas St. thermophilus CH3 were not effectiv e. However, in vitro, both strains prevented DNA damage induced by N-methyl -N'-nitro-N-nitrosoguanidine (MNNG) in isolated primary rat colon cells. Ex tracts prepared from milk fermented with St. thermophilus CH3 were as effic ient in deactivating MNNG as was L-cysteine. Isolated metabolites arising f rom bacteria during fermentation in the colon or in milk [L(+) lactate, D(- ) lactate, palmitic acid and isopalmitic acid] were not effective. We postu late that thiol-containing breakdown products of proteins, via catalysis by bacterial proteases, could be one mechanism by which MNNG or other carcino gens are deactivated in the gut lumen resulting in reduced damage to coloni c mucosal cells.