SELECTION OF AMMONIA-ASSIMILATING BIFIDOBACTERIA AND THEIR EFFECT ON AMMONIA LEVELS IN RAT CECAL CONTENTS AND BLOOD

We selected two B. bifidum strains that could utilise ammonia in preference to any other organic nitrogen compounds by measuring incorporation of the stable isotope [N-15]ammonia by the cells. Their rates of incorporation of [N-15]ammonia were 87.8 per cent for B. bifidum YIT4069 and 81.4 per cent for B. bifidum YIT4070. To investigate the ability of B. bifidum YIT4069 to assimilate ammonia in the intestine, a caecal content incubation system was used. When a fermentable substrate (galactosyllactose) was added to the reaction mixture prepared from the rat caecal contents, the substrate was utilised by bacteria, and the pH of the reaction mixture gradually decreased. This was accompanied by an inhibition of ammonia generation. When B. breve YIT4010, whose growth did not depend on ammonia, was inoculated into the reaction mixture with substrate, the pH of the reaction mixture became reduced immediately, the reduction being greater than in the case of the substrate alone. Ammonia generation was inhibited resulting in maintenance of almost the same concentration of ammonia. When B. bifidum YIT4069 and the substrate were inoculated into the reaction mixture, the pH became reduced and ammonia was simultaneously removed from the system. To assess the effects in vivo B. bifidum YIT4069 and galactosyllactose were injected directly into the caecum. Four hours after the intracaecal injection the ammonia level in the portal vein was significantly lowered. The ammonia levels in the portal vein and abdominal aorta, however, increased when rats were fed a commercial diet containing 8.3 per cent of galactosyllactose for 5 d. This increase was prevented by a 5 d oral administration of B. bifidum YIT4069.