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Injection of a lethal dose of 5-fluorouracil (400 mg/kg) into mice induced an extraordinary increase in the levels of endogenous Escherichia coli in the intestine, systemic translocation of the bacteria to the liver, and loss of body weight. Daily administration of a fermented milk (0.2 ml) containing Bifidobacterium breve strain Yakult, B. bifidum strain Yakult, and Lactobacillus acidophilus strain Yakult (10[sup 8.5-9.5] CFU/ml) to mice for 7 days before and 9 days after 5-FU prevented the endogenous infection, whereas treatment of mice with water or unfermented milk did not lead to any significant protection against the infection. The levels of indigenous bifidobacteria in the intestinal contents decreased markedly with time after injection of 5-FU in the control groups, and this was associated with a marked decrease in the concentration of total short-chain fatty acids (SCFAs) and an increase in the pH on day 10 after 5-FU administration (pH: 7.3, SCFAs: 40 mM). Administration of fermented milk maintained the concentrations of the total SCFAs at the same levels as before 5-FU administration (120 mM) throughout the experimental period. The pH and concentration of acetic acid in the intestinal contents (pH: 6.7, AA: 90 mM) were found to suppress in vitro growth of an indigenous strain of E. coli These results suggest that the protective effect of oral administration of fermented milk against the endogenous E. coli infection resulted from prevention of any abnormal increase in E. coli levels via normalization of the 5-FU-induced disruption of the intestinal environment. [ABSTRACT FROM AUTHOR]