Regulation of lactose, glucose and sucrose metabolisms in S. thermophilus.

Streptococcus thermophilus is a bacterium widely used in the production of yogurts and cheeses, where it efficiently ferments lactose, the saccharide naturally present in milk. It is also employed as a starter in dairy- or plant-based fermented foods that contain saccharides other than lactose (e.g., sucrose, glucose). However, little is known about how saccharide use is regulated, in particular when saccharides are mixed. Here, we determine the effect of the 5 sugars that S. thermophilus is able to use, at different concentration and when they are mixed on the promoter activities of the C-metabolism genes. Using a transcriptional fusion approach, we discovered that lactose and glucose modulated the activity of the lacS and scrA promoters in a concentration-dependent manner. When mixed with lactose, glucose also repressed the two promoter activities; when mixed with sucrose, lactose still repressed scrA promoter activity. We determined that catabolite control protein A (CcpA) played a key role in these dynamics. We also showed that promoter activity was linked with glycolytic flux, which varied depending on saccharide type and concentration. Overall, this study identified key mechanisms in carbohydrate metabolism - autoregulation and partial hierarchical control - and demonstrated that they are partly mediated by CcpA.

Co-utilization of saccharides in mixtures: Moving toward a new understanding of carbon metabolism in Streptococcus thermophilus.

The lactic acid bacterium Streptococcus thermophilus is widely used in food production, notably in yogurt fermentation. It evolved under highly specific ecological conditions, resulting in its ability to efficiently metabolize lactose, the main saccharide in milk. However, when used in sweetened dairy products or plant-based products, S. thermophilus may encounter other saccharides (i.e. alone or in mixtures). To date, the bacterium's metabolic capacities in such contexts have been poorly characterized. Here, we explored saccharide utilization by 39 S. thermophilus strains. Using in silico analysis, we discovered that the identity and structure of saccharide utilization genes are conserved across strains, and we identified six saccharides that might be metabolized. Although underlying genetic variability was low, strains nonetheless displayed differences in growth when supplied with different saccharides: lactose, sucrose, fructose, and glucose. Interestingly, we found that strains preferentially used lactose and sucrose in tandem when given saccharide mixtures. Furthermore, we uncovered some main potential drivers of saccharide metabolism in S. thermophilus. Notably, the sucrose transporter ScrA is also responsible for importing glucose. Overall, this research has yielded useful findings that can help the development of new fermented foods, including plant-based products, in which sucrose may serve as a major carbon source.