GABA production by Streptococcus thermophilus St 8.1 and transcriptional regulation of the GABA biosynthesis key genes

Due to the purported health benefits of γ-aminobutyric acid (GABA) to consumers, the production of this compound by the lactic acid bacterium Streptococcus thermophilus is of applied interest in dairy. The composition and structure of the GABA operon (gadBC) in S. thermophilus are well established, but the dual regulation of gadB and gadC under technological variables remains poorly understood. This study investigated the transcriptional regulation in S. thermophilus St 8.1 of gadB and gadC genes, which encode the glutamate decarboxylase enzyme and the glutamate/γ-aminobutyrate antiporter, respectively. GABA production and the transcription of GABA-related genes were evaluated, respectively, in a rich medium (LM17) and a chemically defined medium (CDM) under conditions resembling those naturally found during milk fermentation and ripening of dairy products. Temperature, pH, and the concentration of monosodium glutamate (MSG), NaCl, and carbohydrates showed distinct effects on the yield of GABA production in LM17. As assessed by quantitative reverse transcription PCR (qRT-PCR), the presence of MSG in CDM (CDM_MSG) led to a differential upregulation of both gadB and gadC genes. Transcription of gadB increased between 4 and 6 h of incubation as the pH dropped below 5.0, reaching a maximum at 8 h (eightfold). In contrast, gadC exhibited a bimodal transcription pattern, with expression peaking at 2 h (2.5-fold) and 8 h (threefold), corresponding to pH values above 5.0 and below 5.0, respectively. Moreover, transcription of gad genes and GABA production in CDM_MSG increased and decreased after supplementation with NaCl and lactose, respectively. The results indicate that gadB transcription is triggered by low pH, while gadC might be subject to catabolic repression. In agreement with the latter, a conserved cre-like sequence was identified upstream of the gadC start codon. A clearer understanding of the regulation of gad genes would facilitate the rational use of S. thermophilus as a starter culture for the production of GABA-enriched fermented dairy products.