Improving gamma-aminobutyric acid production in milk by levilactobacillus brevis

Abstract

Gamma-aminobutyric acid (GABA) acts as a major inhibitory neurotransmitter in central nervous system. GABA and GABA-rich foods have shown multiple pharmaceutical effects in different animal models and clinic trials. Thus, there is an increasing trend to manufacture GABA-rich dairy products serving as highly effective carrier of GABA. Levilactobacillus brevis harboring intact gad operon was regarded as a promising cell factory for GABA production, but the organism was not able to survive in milk because of absence of cell envelope proteinase. Co-culturing non-proteolytic Lb. brevis with Streptococcus thermophilus was proved to improve the viability of Lb. brevis and GABA production in milk. However, there are several research questions to be answered: 1) the interaction mechanism between Lb. brevis and S. thermophilus to produce GABA during milk fermentation; 2) the application of co-culture system in GABA-rich soymilk production; 3) the method to improve GABA content and reduce residual MSG in fermented milk; 4) the mechanism behind cysteine for improving GABA production. Comparative peptidomic and metatranscriptomic analyses were performed to unravel the casein and lactose utilization patterns during milk fermentation with the co-culture (Lb. brevis 145 and S. thermophilus 1275). We found particular peptides hydrolyzed by S. thermophilus were transported and biodegraded with peptidase in Lb. brevis to meet the growth needs of the latter. In addition, amino acid synthesis and metabolism in Lb. brevis were also activated to further support its growth. Glucose, as a product of lactose hydrolysis by S. thermophilus, was outcompeted by Lb. brevis as a main carbon source to produce ATP. In the stationary phase, pH of fermented milk dropped to 4.5 due to accumulation of lactic acid produced by S. thermophilus, genes expression involved in pyridoxal phosphate (coenzyme of glutamic acid decarboxylase) metabolism and glutamic acid decarboxylase (Gad) in Lb. brevis were induced for GABA production under acidic environment. Soymilk rich in peptides was found more suitable than milk casein for GABA production by the co-culture. Importantly, there was a weak correlation between GABA production and sugar utilization. Instead, S. thermophilus produced lactic acid from carbohydrate catabolism to create acidic environment, which influenced GadA activity in Lb. brevis to regulate GABA production during soymilk fermentation. Supplementation with cysteine was found to significantly improve GABA production with a minimum level of residual MSG by the co-culture during milk fermentation. Meanwhile, viable cell counts of Lb. brevis and lactic acid production were increased, and casein hydrolysis pattern was not influenced. Moreover, -SH containing chemicals including cystine, reduced glutathione and oxidized glutathione showed similar effects as cysteine in improving GABA production. Subsequently, the specific mechanism behind cysteine on improving GABA production in Lb. brevis was investigated. Lb. brevis was capable of producing H2O2, cysteine protected Lb. brevis against H2O2-induced oxidative damage to increase cell viability for the enhancement of GABA production. Besides, cysteine promoted glucose consumption to produce acetyl-CoA for synthesizing long-chain fatty acids to significantly up-regulate GABA biosynthesis. Overall, this study systematically explained enhancement of GABA production in milk fermented by Lb. brevis together with S. thermophilus.