Molecular Analysis of Glutamate Decarboxylases in .

() is a common bacterium inhabiting the intestines of humans and other animals. Most strains of this species can produce gamma-aminobutyric acid (GABA) the glutamate decarboxylase (GAD) system, but the presence and genetic organization of their GAD systems are poorly characterized. In this study, our bioinformatics analyses showed that the GAD system in strains was generally encoded by three genes (, , and ), together with an antiporter gene () and regulator gene (), and these genes are organized in a cluster. This finding contrasts with that for other lactic acid bacteria. SDMCC050406, a GABA producer isolated from human feces, was employed to investigate the contribution of the three genes to GABA biosynthesis. The results showed that the relative expression level of was higher than those of and in the exponential growth and stationary phases, and this was accompanied by the synchronous transcription of . After heterologous expression of the three genes in BL21 (DE3), the value of the purified GAD3 was 4.26 ± 0.48 mM, a value lower than those of the purified GAD1 and GAD2. Moreover, gene inactivation caused decreased GABA production, accompanied by a reduction in resistance to acid stress. These results indicated that plays a crucial role in GABA biosynthesis and this property endowed the strain with acid tolerance. Our findings provided insights into how strains survive the acidic environments of fermented foods and throughout transit through the stomach and gut while maintaining cell viability.