In vitro investigations on the impact of fermented dairy constituents on fecal microbiota composition and fermentation activity

Fermented dairy constitutes a major dietary source and contains lactose as the main carbohydrate and living starter cultures, which can encounter the intestinal microbiota after ingestion. To investigate whether dairy-related nutritional and microbial modulation impacted intestinal microbiota composition and activity, we employed static fecal microbiota fermentations and a dairy model system consisting of lactose and Streptococcus thermophilus wild type and β-galactosidase deletion mutant. In addition, we conducted single-culture validation studies. 16S rRNA gene-based microbial community analysis showed that lactose increased the abundance of Bifidobacteriaceae and Anaerobutyricum and Faecalibacterium spp. The supplied lactose was hydrolyzed within 24 h of fermentation and led to higher expression of community-indigenous β-galactosidases. Targeted protein analysis confirmed that bifidobacteria contributed most β-galactosidases together with other taxa, including Escherichia coli and Anaerobutyricum hallii. Lactose addition led to higher (P < 0.05) levels of butyrate compared to controls, likely due to lactate-based cross-feeding and direct lactose metabolism by butyrate-producing Anaerobutyricum and Faecalibacterium spp. Representatives of both genera used lactose to produce butyrate in single cultures. When supplemented at around 5.5 log cells mL⁻¹, S. thermophilus or its β-galactosidase-negative mutant outnumbered the indigenous Streptococcaceae population at the beginning of fermentation but had no impact on lactose utilization and final short-chain fatty acid profiles.