The glutamate decarboxylase (GAD) system of Lactobacillus brevis involves two isoforms of GAD, GadA and GadB, which catalyze the conversion of L-glutamate to γ-aminobutyric acid (GABA) in a proton-consuming reaction contributing to intracellular pH homeostasis. However, direct experimental evidence for detailed contributions of gad genes to acid tolerance and GABA production is lacking. Molecular analysis revealed that gadB is cotranscribed in tandem with upstream gadC, and that expression of gadCB is greatly upregulated in response to low ambient pH when cells enter the late exponential growth phase. In contrast, gadA is located away from the other gad genes, and its expression was consistently lower and not induced by mild acid treatment. Analysis of deletion mutations in the gad genes of L. brevis demonstrated a decrease in the level of GAD activity and a concomitant decrease in acid resistance in the order of wild-type> ΔgadA> ΔgadB> ΔgadC> ΔgadAB, indicating that the GAD activity mainly endowed by GadB rather than GadA is an indispensable step in the GadCB mediated acid resistance of this organism. Moreover, engineered strains with higher GAD activities were constructed by overexpressing key GAD system genes. With the proposed two-stage pH and temperature control fed-batch fermentation strategy, GABA production by the engineered strain L. brevis 9530: pNZ8148-gadBC continuously increased reaching a high level of 104.38 ± 3.47 g/L at 72 h. This is the first report of the detailed contribution of gad genes to acid tolerance and GABA production in L. brevis. Enhanced production of GABA by engineered L. brevis was achieved, and the resulting GABA level was one of the highest among lactic acid bacterial species grown in batch or fed-batch culture.

Dairy products—encompassing yogurt, kefir, cheese, and cultured milk beverages—are emerging as versatile, food-based modulators of gut microbiota and host physiology. This review synthesizes mechanistic insights demonstrating how live starter cultures and their fermentation-derived metabolites (short-chain fatty acids, bioactive peptides, and exopolysaccharides) act synergistically to enhance microbial diversity, reinforce epithelial barrier integrity via upregulation of tight-junction proteins, and modulate immune signaling. Clinical evidence supports significant improvements in metabolic parameters (fasting glucose, lipid profiles, blood pressure) and reductions in systemic inflammation across metabolic syndrome, hypertension, and IBS cohorts. We highlight critical modulatory factors—including strain specificity, host enterotypes and FUT2 genotype, fermentation parameters, and matrix composition—that govern probiotic engraftment, postbiotic yield, and therapeutic efficacy. Despite promising short-term outcomes, current studies are limited by heterogeneous designs and brief intervention periods, underscoring the need for long-term, adaptive trials and integrative multi-omics to establish durability and causality. Looking forward, precision nutrition frameworks that harness baseline microbiota profiling, host genetics, and data-driven fermentation design will enable bespoke fermented dairy formulations, transforming these traditional foods into next-generation functional matrices for targeted prevention and management of metabolic, inflammatory, and neuroimmune disorders.