Metabolic interactions and growth dynamics of Bifidobacterium strains in response to human milk oligosaccharides and lactose.

Bifidobacteria are major representatives of the intestinal microbiota in infants typically identified as characteristic of healthy breastfed infants. Their establishment is driven by the presence of carbohydrates, namely human milk oligosaccharides (HMOs). Metabolic products of HMO utilization by the infant microbiota, such as short chain fatty acids and lactate, are associated with health benefits for the host. In the present study, we investigated the growth and acidic metabolites produced by infant typical Bifidobacterium strains B. longum subsp. infantis, B. breve and B. bifidum as single cultures as well as in co-cultures. The HMOs, 2'-fucosyllactose (2'-FL) and 6'-sialyllactose (6'-SL) were used as carbohydrate substrates given separately and together or as a mix with lactose and were compared with lactose alone and glucose as standard carbohydrate supplements in media. Bacterial growth of single cultures and co-cultures showed differences in growth curve and optical density at 36 h depending on carbohydrates. The best growth was reached in media supplemented with equimolar amounts of 2'-FL and lactose and accompanied by high concentrations of organic acids in all single and co-cultured Bifidobacterium strains as determined by HPLC analysis. When 6'-SL was supplemented, B. bifidum was the only strain able to grow in single cultures. In co-cultures, however, its presence affected the growth curves of mixed cultures although its own proportion of cells after 36 h remained as low to that of an estimated 10% of all bifidobacterial cells as determined by qPCR. The production of bacterial metabolites was more complex with 6'-SL and in combination with 2'-FL than with any other carbohydrate substrate. These results indicate that bacterial growth on specific HMO substrates in single strain cultures is not predictive of the potential production of organic acids in mixed strain cultures. The interaction of multiple Bifidobacterium strains and their cross-feeding properties when exposed to specific substrate compositions may affect the production of key metabolites important for infants and in later life.