Comparative genomics of Lactobacillus brevis uncovers its common capability for efficiently synthesizing neuroactive γ-aminobutyric acid

Abstract

γ-Aminobutyric acid (GABA) is the chief inhibitory neurotransmitter in mammalian central nervous system and has shown anti-hypertensive and anti-depressant activities to the host after oral administration. However, its content in natural animal and plant products is too low to deliver benefits to human. Thus, GABA synthesized by food-grade bacteria such as Lactobacillus and Bifidobacterium is an important source and its producers could be used for manufacturing GABA-rich fermented dairy foods. Many GABA-producing Lactobacillus and Bifidobacterium strains have been isolated and characterized in the last decade and have shown strain-specific capability in the synthesis of GABA. Among these GABA producers, Lactobacillus brevis seems to be the most common cell factory for synthesizing GABA. In this study, comparative genomic approach was used to identify which LAB species have the common ability to produce high amount of GABA and to identify the essential genetic elements for GABA production. It was found that gene encoding glutamic acid decarboxylase (GAD) and an intact gad operon were present in all the sequenced strains of L. brevis at the species level, but not all the strains of other Lactobacillus and Bifidobacterium species possess an intact gad operon including a regulator gadR, a gadA- or gadB-encoding GAD, and an antiporter gadC. This suggests the common capability of L. brevis to synthesize GABA. Moreover, enzyme assay for two GADs from L. brevis indicated that both enzymes are functional with high activities. Carbohydrate utilization by model strain Lb. brevis NPS-QW-145 generated different lactic acid production, which showed strong positive correlation with its GABA yields suggesting that intracellular lactic acid production triggers its GABA biosynthesis, which was also evidenced by the intracellular pH level of the cells. Moreover, among all of acid resistance (AR) pathways in Lb. brevis, GAD pathway contributed to late acid resistance whereas tyrosine decarboxylation (TDC) and arginine deimination (ADI) pathways were activated during lag and log phases, which were confirmed by transcriptional profiles and concentrations of the end metabolites of each AR. The present study highlights the common capability of Lb. brevis for highly efficient biosynthesis of GABA.