Effects of Acid Stress on the Expression of Key Genes of Glutamate Decarboxylase System and Enzyme Activity in Lactobacillus Brevis

Glutamate decarboxylase(GAD) can catalyze the decarboxylation reaction of L-glutamate(Glu) to produce γ-aminobutyric acid(GABA). The synergetic action of GAD and membrane-located Glu/GABA antiporter is related to the acid resistance of some bacterial cells under acidic conditions. The GAD system of Lactobacillus brevis is mainly comprised of two glutamate decarboxylases(Gad A and Gad B) and a Glu/GABA antiporter(Gad C). In this study, quantitative real-time PCR was employed to investigate the effect of acid stress on the expression of gad A, gad B and gad C in L. brevis CGMCC NO.1306 under different culture conditions. The results indicate that gad B forms an operon with gad C and the expression of gad CB is greatly upregulated in response to low ambient p H when cells enter late exponential growth phase. The GAD activity of cells is mainly from Gad B and the highest enzyme activity is observed in the stationary growth phase. In contrast, the expression of gad A is almost not affected by the ambient p H and cell cycle. These results will provide guidance for the optimization of GABA fermentation as well as the study on the ability of acid resistance of L. brevis under acid stress.