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The relevance of carbon dioxide metabolism in Streptococcus thermophilus

¹ Department of Food Science and Microbiology, Milan, Italy
² Istituto Sperimentale Italiano Lazzaro Spallanzani, sezione di Proteomica, Facoltà di Medicina Veterinaria, Milan, Italy
³ Proteomics and Mass Spectrometry Laboratory, ISPAAM, National Research Council, Naples, Italy
⁴ Department of Veterinary Pathology, Hygiene and Public Health, University of Milan, Milan, Italy
⁵ IRCCS Policlinico San Matteo, Pavia, Italy

Streptococcus thermophilus is a major component of dairy starter cultures used for the manufacture of yoghurt and cheese. In this study, the CO₂ metabolism of S. thermophilus DSM 20617^T, grown in either a N₂ atmosphere or an enriched CO₂ atmosphere, was analysed 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, CO₂ 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 N₂ showed a significantly higher milk acidification rate compared with those grown in an enriched CO₂ atmosphere. Experiments carried out on S. thermophilus wild-type and its derivative mutant, which was inactivated in the phosphoenolpyruvate carboxylase and carbamoyl-phosphate synthase 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 micro-organism, underlining the essential role of CO₂ in S. thermophilus physiology, and suggesting potential applications in dairy fermentation processes.

Correspondence
Diego Mora
diego.mora{at}unimi.it

The GenBank/EMBL/DDBJ accession number for the sequence determined in this work is AM983544.