Lactic acid bacteria (LAB) are a heterogeneous group of Gram-positive facultative anaerobic microorganisms

Gram-positive facultative anaerobic microorganisms that produce large amounts of lactic acid. LAB, which are seen in milk and the human intestine, are used for the manufacture of yogurt and cheese. They are referred to as animal-derived LAB. On the other hand, LAB exist on the surface of fruits, vegetables, and flowers. These LAB, designated plant-derived LAB, have higher adaptation characteristics for withstanding harsh conditions like plant stems and leaf surfaces. The bacteria are useful to make traditional fermented foods, such as pickles, soy sauce, ‘miso,’ and ‘kimchi.’ Thus, LAB are roughly classified into two groups according to differences in their growth environment. The characteristics of plant-derived LAB should be distinguishable from those of animal-derived LAB. Recently, LAB are receiving attention for their probiotic potential for humans. Gamma-aminobutyric acid (GABA), a four-carbon nonprotein amino acid, is produced by animals and plants. Being an analog of glutamic acid, GABA has been reported to possess several physiological functions such as neurotransmission, tranquilization, antihypertension, prevention of diabetic condition, and diuretic effects. Now, several functional foods supplemented with GABA are produced to satisfy the market demand. GABA is biosynthesized by microorganisms such as Lactobacillus (Lb.) paracasei, Lb. brevis, and Streptococcus salivarius via decarboxylation of glutamic acid by glutamate decarboxylase (EC 4.1.1.15) which is a pyridoxal phosphate-dependent enzyme. Due to the commercial demand of GABA, the screening of bacteria producing a large amount of the amino acid and the improvement of the efficient production system are being investigated actively. The production of GABA by a batch fermentation system has been reported using various microorganisms, but the productivity has remained on a low level, and the manufacturing costs continue to be high. In the present study, we successfully isolated carrot leafderived LAB, which produce GABA from monosodium Lglutamate (L-MSG) at a hyper conversion rate. The GABA producer, which is designated G-15, was identified as Enterococcus (E.) avium. The optimum culture conditions for a high production of GABA were found to involve a continuous L-MSG feeding method using a jar-fermentor with E. avium G-15, suggesting that E. avium G-15 displays a great potential ability for the commercial-level fermentation production of GABA. A functional analysis of genes for GABA hyper-production was also conducted.