We investigated the adaptation to milk of Streptococcus thermophilus LMG18311 using a proteomic approach. Two-dimensional electrophoresis of cytosolic proteins were performed after growth in M17 medium or in milk. A major modification of the proteome concerned proteins involved in the supply of amino acids, like the peptidase PepX, and several enzymes involved in amino acid biosynthesis. In parallel, we observed the upregulation of the synthesis of seven enzymes directly involved in the synthesis of purines, as well as formyl-tetrahydrofolate (THF) synthetase and serine hydroxy-methyl transferase, two enzymes responsible for the synthesis of compounds (THF and glycine, respectively) feeding the purine biosynthetic pathway. The analysis also revealed a massive increase in the synthesis of pyruvate formate-lyase (PFL), the enzyme which converts pyruvate into acetyl coenzyme A and formate. PFL has been essentially studied for its role in mixed-acid product formation in lactic acid bacteria during anaerobic fermentation. However, formate is an important methyl group donor for anabolic pathway through the formation of folate derivates. We hypothesized that PFL was involved in purine biosynthesis during growth in milk. We showed that PFL expression was regulated at the transcriptional level and that pfl transcription occurred during the exponential growth phase in milk. The complementation of milk with formate or purine bases was shown to reduce pfl expression, to suppress PFL synthesis, and to stimulate growth of S. thermophilus. These results show a novel regulatory mechanism controlling the synthesis of PFL and suggest an unrecognized physiological role for PFL as a formate supplier for anabolic purposes.The thermophilic bacteria Streptococcus thermophilus is one of the most widely used lactic acid bacteria (LAB) in the dairy fermentation industry for yoghurt and cheese production. In the industrial implementation of S. thermophilus, fast-growing capacity is crucial to enable rapid and intense acidification of milk. Identification of functions activated during growth in milk should help to understand the molecular adaptation of this bacterium to milk and provide the basis for targeted strain selection.In contrast with other LAB, the only environment from which S. thermophilus has been isolated is milk (38). In line with this restricted ecological niche, lactose, not glucose, is the preferred carbon source for S. thermophilus (31). The capacity to ferment lactose, the main sugar of milk, into lactic acid is essential for growth in milk and depends on a non-phosphotransferase system lactose permease (LacS) and a beta-galactosidase (LacZ) (40). Concerning the capacity of S. thermophilus to fulfill its amino acid needs during growth in milk, the species displays only a few amino acid auxotrophies compared to the model LAB, L. lactis (29,33). The growth of Lactococcus lactis in milk depends largely on the activity of a cell-wallbound proteinase (PrtP) that is responsible for casein hydrolysis, the main source of amino a...