Effect of fumaric acid on the growth of Lactobacillus delbrueckii ssp. bulgaricus during yogurt fermentation

Yogurt is traditionally fermented by a symbiotic starter culture of Streptococcus thermophilus and Lac-tobacil lus delbrueckii ssp. bulgaricus. These bacteria exchange metabolites with each other to meet their nutritional demands during protocooperation, resulting in a shorter fermentation time. In this study, we inves-tigated whether fumaric acid functions as a symbiotic agent to promote the growth of Lb. bulgaricus by evalu-ating 8 strains of Lb. bulgaricus and 7 strains of Strep. thermophilus. All the tested Lb. bulgaricus strains me -tab olized the added fumaric acid into succinic acid dur-ing monoculture in milk, and 6 strains (75%) showed shorter fermentation time compared with the control. The addition of malic acid showed similar trends as that of fumaric acid, indicating that the reverse tricar-boxylic acid cycle was functioning in Lb. bulgaricus. All 7 Strep. thermophilus strains tested produced fumaric acid during monoculture in milk. Further, in Lb. bul-garicus 2038, the gene expression of fumarate reductase that converts fumaric acid to succinic acid, was higher in the coculture with Strep. thermophilus 1131 than in the monoculture. These findings indicate that fumaric acid produced by Strep. thermophilus can function as a symbiotic substance during yogurt fermentation to stimulate the growth of Lb. bulgaricus.