Shiga toxin-producing Escherichia coli and Salmonella, causative bacteria of food poisoning (intestinal infectious disease) in humans, are still serious problems. Shiga toxin-producing E. coli O157:H7 (STEC) grows and produces Shiga toxin (Stx) in the intestine, and causes hemorrhagic enteritis. A typical etiologic agent of Salmonella food poisoning, Salmonella enterica serovar Typhimurium (S. Typhimurium), grows in the intestine and invades the body via the intestinal epithelium, causing inflammation. The importance of the prevention of STEC- and S. Typhimurium-induced food poisoning has been stressed because they frequently cause outbreaks, the course is rapid, and only a very small number of bacteria (10¹ to 10³ CFU) is needed to induce a severe infection. Probiotics are defined as `Live microorganisms which when administered in adequate amounts confer a health benefit on the host'. Bifidobacteria, major constituents of the intestinal flora, are typical probiotics which are expected to help prevent intestinal infection. In this study, we investigated the anti-infectious activity of bifidobacteria against STEC and S. Typhimurium infections using a mouse intestinal infection model, and analyzed the infection-preventive mechanism. For STEC infection, a novel mouse fatal infection model was prepared by combining STEC infection at 5 × 10 ³ CFU and Mitomycin C (MMC) treatment (for the induction of stx gene expression) in the late logarithmic phase of intestinal STEC growth. The anti-infectious activity of the orally administered probiotic Bifidobcterium breve strain Yakult (BbY) was investigated using this mouse intestinal STEC infection model. STEC-induced death was strongly inhibited in BbY-treated mice. Interestingly, STEC growth in the intestine was not inhibited, but stx gene expression and Stx production were strongly inhibited. In addition, the intestinal environment was improved in the BbY-treated mice through normalization of the intestinal level of acetic acid, a major organic acid in the intestine, and pH. When STEC was grown in vitro in a medium reproducing the acetic acid level and pH in the cecum, Stx production was completely inhibited, suggesting that the expression of this pathogenic factor was inhibited by BbY-induced improvement of the intestinal environment. In the mouse intestinal S. Typhimurium infection model, BbY inhibited the abnormal growth of S. Typhimurium and improved the intestinal environment, resulting in the inhibition of systemic S. Typhimurium infection. This study, using an experimental animal model, clarified the preventive effect of the probiotic BbY on food poisoning (intestinal infectious disease) caused by STEC and S. Typhimurium. Improvements of the intestinal environment, such as elevation of the acetic acid concentration and decrease in pH level, induced by intestinal BbY colonization, are suggested to be important defense mechanisms for the inhibition of pathogenic factors production induce by intestinal STEC and retardation of intestinal S. Typhimurium growth. These findings suggest the usefulness of probiotics as a novel preventive agent for human food poisoning (intestinal infectious disease).