AbstractReinforcement of hydroperoxide-eliminating activity in the intestines and colon should prevent associated diseases. We previously isolated a lactic acid bacterium,Pediococcus pentosaceusBe1, that facilitates a 2-electron reduction of hydrogen peroxide to water. In this study, we successfully isolated an alternative lactic acid bacterium,Lactobacillus plantarumP1-2, that can efficiently reduce environmental alkyl hydroperoxides and fatty acid hydroperoxides to their corresponding hydroxy derivatives through a 2-electron reduction. Each strain exhibited a wide concentration range with regard to the environmental reducing activity for each hydroperoxide. Given this, the two lactic acid bacteria were orally administered to the oxygen-sensitive short-lived nematode mutant, and this resulted in a significant expansion of its lifespan. This observation suggests thatP. pentosaceusBe1 andL. plantarumP1-2 inhibit internal oxidative stress. To determine the specific organs involved in this response, we performed a similar experiment in rats, involving induced lipid peroxidation by iron-overloading. We observed that onlyL. plantarumP1-2 inhibited colonic mucosa lipid peroxidation in rats with induced oxidative stress.