Catalase and subtilisin have been proposed as the active molecules responsible for the effects of B, subrilus (natto) on intestinal microfiora. The growth of three strains of lactobacilli co-cultured aerobically with B subtilis (natto) has been exam Addition of B, subtilis (natta) to the culture medium in vitro resulted in increase in the number of viable cells of all lactobacilli tested. Both catalase and B subtilis (natto) enhanced the growth of Lactobacillus reuteri, whereas B. subtilis (natto), but not catalase, enhanced the growth of Lactobacillus acidophilus. In a medium containing 0.1 mM hydrogen peroxide, its toxic effect on Lb. reuteri was abolished by catalase or B. subtilis (natto). Catalase has been reported to exhibit a growth-promoting effect on lactobacil The ability of lactobacilli readily decreases in the presence of active oxygen species. The decrease of viable cell number is partly attributable to the fact that lactobacilli do not generally produce a defense molecule against active oxygen species. However, aerobic bacteria, includ- ng B. subtilis (natto), can produce catalase. Vegetative cells of B. subtilis primarily produce catalasc-1 in the logarithmic phase of growth. and additionally produce catalase-2 and -3 as growth progresses. s' ntact B. subtilis spores contain only Escherichia coli, Bacteroides spp., and Eubacterium spp., also produce catalase. to It may be important for these bacteria to scavenge hydrogen peroxide in order a colonize the intestine where active oxygen species are produced. The addition of serine protease, subtilisin, from Bacillus licheniformis to the culture medium improved the growth and viability of Lb reuteri and Lb acidophilus in the absence of hydrogen 47 B. subtilis (natto) secretes two serine proteases, subtilisin NAT with an peroxide. 8.7 and a 90-kDa serine proteinase (pl 3.9).56 isoelectric point (pl) of (natto) can enhance the growth Taken together, these results in that B. subtilis and subtilisin. and/or of lactobacilli possibly through production of catalase and subtilisin.