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In breast-fed newborn infants, a stable microflora having more than 90% Bifidobacterium is usually developed in the colon and the feces within 5 days after birth. As a result, pH value lowers and putrefactive bacteria decrease in the feces. In bottle feeding with the milk containing a certain quantity of lactulose, it is observed that pH value, the population of Bifidobacterium, and lysozyme activity in the feces approach the levels of breast feeding. The ratio of Bifidobacterium to the total anaerobic bacteria, lysozyme activity, and concentration of the organic acids were increased while pH was lowered in the feces of infants who were fed the follow-up formula food containing lactulose. Composition of the organic acids in the feces varied depending upon the age. The values of molar ratio of acetic acid to lactic acid in feces of suckling infants were 2.7 to 5.3 and they were elevated by 5-to 7-fold in those feces of weaning infants, while lactic acid was not detected in the feces of the adult subjects. In the in vitro experiments, all bifidobacterial species of the present study assimilated lactulose. The activity for lactulose assimilation was, on the contrary, deficient in Clostridium dificile. When C. perfringens or E. coli was cultured simultaneously with the Bifidobacterium in the PYF culture medium containing lactulose, the former two microbials were suppressed remarkably. Infant rats were inoculated with C. perfringens and fed the milk containing lactulose. The occurrence number of the rats in which C. perfringens was detected in the colon also tends to decline during the feeding.

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