Abstract
The inhibitory effects of seven human-derived Bifidobacterium strains on pathogenic Escherichia coli serotype O-111 were examined in co-cultivation. All of the Bifidobacterium strains tested inhibited the growth of E. coli and limited the increase in the ammonia concentration in the co-cultures. Also, in each instance, the pH of the culture medium decreased due to their production of acetic and lactic acids. From the results of pH-controlled fermentation, the growth inhibition of E. coli was attributed mainly to the decrease in the pH of the culture medium. The possibility that some unidentified inhibitory substances derived from Bifidobacterium strains were present was little thought. In mono-cultures of Bifidobacterium strains, the ammonia concentration in the culture medium decreased to a level below that initially present in the medium prior to inoculation. Enzymatic assays showed that the activity of an enzyme involved in ammonia production (L-asparagine deaminase) was much weaker in each of the Bifidobacterium strains than in E. coli, whereas the activities of enzymes involved in ammonia assimilation (glutamine synthetase, glutamate synthase and glutamate dehydrogenase) were much higher in bifidobacteria. The decrease in the ammonia concentration in the co-culture appears to be related not only to the number of colonyforming units of bifidobacteria but also to the differences among the bacteria examined in enzymatic activities which are concerned with utilization and production of ammonia.
