BioDairy
Lactose is the major carbohydrate of dairy. In the gastrointestinal tract, undigested lactose is utilized by intestinal microbes to produce short chain fatty acids (SCFA), which are considered beneficial for health. Concurrent ingestion of dairy starter cultures can enhance intestinal lactose-utilization and microbial cross-feeding activities, but competitive potential of starter cultures has been investigated by few studies. Employing fecal microbiota fermentations and model dairy systems composed of lactose and ß-galactosidase positive and negative Streptococcus thermophilus in combination with microbiota community and fermentation metabolite analysis and enzyme activity assays and characterisation, we observed that lactose addition increased the abundance of Bifidobacteriaceae, and of Veillonellaceae and Enterobacteraceae in selected samples. The supplied lactose increased expression of community indigenous ß-galactosidases and was hydrolyzed within 24 h of fermentation. Lactose addition led to 1.1-1.8 fold higher levels of butyrate compared to controls likely due to lactate-crossfeeding and to direct lactose metabolism by butyrate producing Anaerobutyricum and Faecalibacterium. When supplemented at around 5.5 log cells mL-1 (total bacteria counts were 7.0 log cells mL-1), S. thermophilus or its ?LacZ mutant outnumbered the indigenous Streptococcaceae population at the beginning of fermentation but had no impact on lactose utilization and final SCFA profiles. Our results suggest that the indigenous microbiota outcompeted the incoming starter culture due to multiple and concurrently occuring strategies to utilize dairy derived lactose. In addition, lactose utilization led to consistent shift of fermentation profiles indicated by higher levels of butyrate, which is considered beneficial for human health.