EFFECT OF DILUTION RATE AND CARBON AVAILABILITY ON BIFIDOBACTERIUM BREVE FERMENTATION

Bifidobacterium breve NCFB 2257 was grown in glucose-limited and nitro gen (N)-limited chemostats at dilution rates (D) from 0.04 to 0.60 h(- 1), to study the effect of nutrient availability on carbohydrate metab olism. The results showed that D had little effect on fermentation pro duct formation, irrespective of the form of nutrient limitation. Howev er, marked differences were observed in the distribution of fermentati on products, that were attributable to glucose availability. In glucos e-limited cultures, formate and acetate were the principal end-product s of metabolism. Lactate was never detected under these, growth condit ions. In contrast, lactate and acetate were mainly formed when glucose was in excess, and formate was not produced. These results are explai ned by the metabolic fate of pyruvate, which can be dissimilated by ei ther phosphoroclastic cleavage to acetyl phosphate and formate, or alt ernatively, it may be reduced to lactate. Enzymic studies were made to establish the mechanisms that regulated pyruvate metabolism. The data demonstrated that control was not exercised through regulation of the synthesis and activity of lactate dehydrogenase (LDH), phosphofructok inase or alcohol dehydrogenase. It is possible however, that there was competition for pyruvate by LDH and the phosphoroclastic enzyme, whic h would determine the levels of lactate and formate produced respectiv ely. These results demonstrate the metabolic flexibility of B. breve, which preferentially uses lactate as an electron sink during N-limited growth, whereas under energy-limitation, carbon flow is directed towa rds acetyl phosphate to maximise ATP synthesis.