The Relevance of Carbon Dioxide Metabolism in Streptococcus thermophilus.

Streptococcus thermophilus is a major component of dairy starters used for the manufacture of yoghurt and cheese. In this study, the CO2 metabolism of S. thermophilus DSM 20617T grown either in a nitrogen or an enriched carbon dioxide atmosphere was analyzed using both genetic and proteomic approaches. Growth experiments performed in a chemically defined medium revealed that CO2 depletion resulted in bacterial arginine, aspartate and uracil auxotrophy. Moreover, carbon dioxide depletion governed a significant change in cell morphology and a high reduction in biomass production. A comparative proteomic analysis revealed that cells of S. thermophilus showed a different degree of energy status depending on the CO2 availability. In agreement with proteomic data, cells grown under nitrogen showed a significantly higher milk acidification rate compared to those grown in enriched carbon dioxide atmosphere. Experiments carried out on S. thermophilus wild-type and its derivative mutant inactivated in the phosphoenolpyruvate carboxylase and carbamoylphosphate synthethase activities, responsible for fixing CO2 to organic molecules, suggested that the anaplerotic reactions governed by these enzymes have a central role in bacterial metabolism. Our results reveal the capnophilic nature of this microorganism, underlining the essential role of carbon dioxide in S. thermophilus physiology and suggesting potential applications in dairy fermentation processes.