Abstract

Gamma-Aminobutyric acid (GABA), a non-protein amino acid, is mainly found in the brain and regulates vertebrate physiological and psychological behaviors such as anxiety and depression blood pressure and hormone secretion. The synthesis of GABA in the brain decreases with age, especially in elders. Hence, there has been increasing interest in use of probiotics for GABA production. 

In this study, several GABA-producing LAB isolates have been isolated from naturally fermented foods such as Korean kimchi. Previous screening methods are time-consuming and inefficient. In the present study, we have developed a novel screening and identification method for GABA-producing LAB from Korean kimchi. Acid treatment was applied to screening procedure to obtain acid-tolerant LAB isolates, and then a simple identification of GABA-producing LAB based on release of gas by these bacteria has been developed. The amount of GABA produced by LAB isolates at various monosodium glutamate (MSG) concentrations and incubation time in MRS medium was quantified by HPLC. Genetic identification of high GABA-producing LAB was carried out by both 16S rRNAgene and glutamate decarboxylase gene.Nine potential GABA-producing LAB isolates were selected by observing gas release during fermentation. The conversion ability of MSG into GABA for all nine LAB isolates was 100% (supplementation level 10 g/L MSG, incubation time 24 h), over 80% (supplementation level 30 g/L MSG, incubation 48 h), over 60% (supplementation level 50 g/L MSG, incubation time 72 h) and over 50% (supplementation level 70 g/L MSG, incubation time 72 h). These nine LAB isolates were genetically identified as Lactobacillus brevis by 16S rRNAgene and confirmed by glutamate decarboxylase gene.

The génome of this bacterium has been completely sequenced in our lab. The absence of genes encoding extracellular or cell wall-anchored proteinases in above sequenced GABA-producing L. brevis strains suggested that Lb. brevis of plant origin may not be able to survive in milk environment and may not be able to produce GABA. We hypothesized that dairy starters could support the growth of non-proteolytic Lb. brevis by producing growth factors, especially amino acids and peptides, thus improving the growth of  Lb. brevis and GABA biosynthesis.Two GAD genes, gadA and gadB, were identified in L. brevis NPS-QW-14. Dairy S. thermophilus, but not L. bulgaricus, improved the viability of L. brevis NPS-QW-145 in milk Dairy S. thermophilus metabolized more glutamate supplemented to milk than L. bulgaricus Certain S. thermophilus strains improved GABA yield from L. brevis NPS-QW-145 and certain S. thermophilus strains enhanced gadA mRNA level in L. brevis NPS-QW-145