Fast-acidification promotes GABA synthesis in response to acid stress in Streptococcus thermophilus

Abstract

Most Streptococcus thermophilus strains possess the conserved gadBC gene cluster for gamma-aminobutyric acid (GABA) synthesis, endowing yogurt with special beneficial characteristics. Here, we found that GABA yields of S. thermophilus were always positively correlated with fast-acidification. To explore the regulation mechanism of acid production to GABA synthesis, the activities of β-galactosidase (LacZ), lactate dehydrogenase (Ldh) and glutamate decarboxylase (GAD) were determined in strains SDMCC050243 and SDMCC050242. The results showed the higher enzyme activities of LacZ and Ldh at the exponential growth phases, the higher GAD activity. Furthermore, the activity of PgadB promoter responding to the transcription of gadB gene increased with the decrease of pH, suggesting that acid production activated the expression of GAD to synthesize GABA. Meanwhile, the viability of S. thermophilus increased with the accumulation of GABA, confirming the protection role of GABA synthesis against acid stress. In milk fermentation using ten GABA-producing strains, the more GABA yields were always accompanied with the shorter curd time, further illustrating that fast-acidification was beneficial to GABA synthesis. Therefore, this study revealed that fast-acidification promoted GABA biosynthesis in S. thermophilus, subsequently protected strains against acid stress, providing theoretical foundations for making the GABA-enriched yogurt.