Meta-transcriptomic Analyses Reveal Improved Gamma-amino butyric acid Production Machinery in Levilactobacillus brevis NPS-QW 145 Co-cultured with Streptococcus thermophilus ASCC1275 during Milk Fermentation

Purpose: High ?-aminobutyric acid (GABA)-producing Levilactobacillus brevis strain NPS-QW 145 along with Streptococcus thermophilus (one of the two starter bacteria used to make yogurt for its proteolytic activity) to enhance GABA production in milk. But a mechanistic understanding on how Levilactobacillus brevis cooperated with S. thermophilus to stimulate GABA production has been lacking. Method: Metatranscriptomic analyses combined with peptidomics were carried out to unravel the casein and lactose utilization patterns during milk fermentation with the co-culture. Results: We found particular peptides hydrolyzed by S. thermophilus 1275 were transported and biodegraded with peptidase in Lb. brevis 145 to meet the growth needs of the latter. In addition, amino acid synthesis and metabolism in Lb. brevis 145 were also activated to further support its growth. Glucose, as a result of lactose hydrolysis by S. thermophilus 1275, but not available lactose in milk, was outcompeted by Lb. brevis 145 as a main carbon source for glycolysis to produce ATP.In the stationary phase, under the acidic condition due to accumulation of lactic acid produced by S. thermophilus 1275, genes expression involved in pyridoxal phosphate (coenzyme of glutamic acid decarboxylase) metabolism and glutamic acid decarboxylase (Gad) in Lb. brevis 145 were induced for GABA production. Overall design: 6 samples; triplicates; Control group (S): mRNA profiles of 18-h fermented milk inoculated with Streptococcus thermophilus; Experimental group (SL): mRNA profiles of 18-h fermented milk inoculated with Streptococcus thermophilus and Levilactobacillus brevis strain NPS-QW 145