Streptococcus thermophilus , an extensively used lactic starter, is generally produced in yeast extract-based media containing a complex mixture of peptides whose exact composition remains elusive. In this work, we aimed at investigating the peptide content of a commercial yeast extract (YE) and identifying dynamics of peptide utilization during the growth of the industrial S. thermophilus N4L strain, cultivated in 1 l bioreactors under pH-regulation. To reach that goal, we set up a complete analytical workflow based on mass spectrometry (peptidomics). About 4,600 different oligopeptides ranging from 6 to more than 30 amino acids in length were identified during the time-course of the experiment. Due to the low spectral abundance of individual peptides, we performed a clustering approach to decipher the rules of peptide utilization during fermentation. The physicochemical characteristics of consumed peptides perfectly matched the known affinities of the oligopeptide transport system of S. thermophilus . Moreover, by analyzing such a large number of peptides, we were able to establish that peptide net charge is the major factor for oligopeptide transport in S. thermophilus N4L.
Despite their importance as potent odors that contribute to the aroma of numerous cheeses, S-methyl thioesters formation pathways have not been fully established yet. In a first part of our work, we demonstrated that Brevibacterium antiquum and Brevibacterium aurantiacum could produce S-methyl thioesters using short-chain fatty acids or branched-chain amino acids as precursors. Then, we focused our work on L-leucine catabolism using liquid chromatography tandem mass spectrometry and gas chromatography-mass spectrometry analyses coupled with tracing experiments. For the first time, several acyl-CoAs intermediates of the L-leucine to thioesters conversion pathway were identified. S-methyl thioisovalerate was produced from L-leucine, indicating that this amino acid was initially transaminated. Quite interestingly, data also showed that other S-methyl thioesters, e.g., S-methyl thioacetate or S-methyl thioisobutyrate, were produced from L-leucine. Enzymatic and tracing experiments allowed for postulating catabolic pathways leading to S-methyl thioesters biosynthesis.
With the view to investigate the presence of thiols in cheese, the use of different methods of preparation and extraction with an organomercuric compound ( p-hydroxymercuribenzoate) enabled the isolation of a new compound. The analysis of cheese extracts by gas chromatography coupled with pulse flame photometry, mass spectrometry, and olfactometry detections led to the identification of ethyl 3-mercaptopropionate in Munster and Camembert cheeses. This compound, described at low concentrations as having pleasant, fruity, grapy, rhubarb, and empyreumatic characters, has previously been reported in wine and Concord grape but was never mentioned before in cheese. A possible route for the formation of this compound in relation with the catabolism of sulfur amino acids is proposed.
Peptides present in growth media are essential for nitrogen nutrition and optimal growth of lactic acid bacteria. In addition, according to their amino acid composition, they can also directly or indirectly play regulatory roles and influence the global metabolism. This is especially relevant during the propagation phase to produce high cell counts of active lactic acid bacteria used as starters in the dairy industry. In the present work, we aimed at investigating how the respective compositions of two different yeast extracts, with a specific focus on peptide content, influenced Streptococcus thermophilus metabolism during growth under pH-controlled conditions. In addition to free amino acids quantification, we used a multi-omics approach (peptidomics, proteomics and transcriptomics) to identify peptide initially present in the two culture media, and to follow S. thermophilus gene expression and bacterial protein production during growth. The free amino acid and peptide composition of the two yeast extracts differed qualitatively and quantitatively. Nevertheless, the two yeast extracts sustained similar growth of S. thermophilus and led to equivalent final biomasses. However, transcriptomics and proteomics showed differential gene expression and protein production in several S. thermophilus metabolic pathways, especially amino acid, citrate, urease, purine and pyrimidine metabolisms. The probable role of the regulator CodY is discussed in this context. Moreover, we observed significant differences in the production of regulators and of a quorum sensing regulatory system. The possible roles of yeast extract peptides on the modulation of the quorum sensing system expression are evaluated. Importance Improving the performance and industrial robustness of bacteria used in fermentations and food industry remains a challenge. We showed here that two Streptococcus thermophilus fermentations, performed with the same strain in media varying only by their yeast extract composition and more especially their peptide contents, led to similar growth kinetics and final biomasses but several genes and proteins were differentially expressed/produced. In other words, subtle variations in peptide composition of the growth medium can finely tune the metabolism status of the starter. Our work therefore suggests that acting on growth media components and especially on their peptide content, we could modulate bacterial metabolism and produce bacteria differently programmed for further purposes. This might have applications for preparing active starter cultures.
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