Synergistic Enhancement of Fecal GABA Content by Bifidobacterium adolescentis 4-2 and Mannooligosaccharides in a Human Intestinal Flora Model

Background: Gamma-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the central nervous system and exerts multiple health-promoting effects, including antidepressant, hypotensive, immunomodulatory, and antidiabetic actions. Previous studies have reported a positive correlation between the abundance of Bifidobacterium species in the gut and fecal GABA levels. However, the direct contribution of specific GABA-producing Bifidobacterium strains and the associated interaction with prebiotics remain insufficiently characterized. Methods: This study aimed to elucidate the direct effects of a GABA-producing strain, Bifidobacterium adolescentis 4-2, in combination with the prebiotic mannooligosaccharides (MOS), on fecal GABA concentrations using the Kobe University Human Intestinal Microbiota Model (KUHIMM), an in vitro simulation of the human colonic microbiota. GABA levels were quantified via high-performance liquid chromatography, and changes in microbial composition following oligosaccharide supplementation were assessed by next-generation sequencing of 16S rRNA genes. The presence and activity of the β-mannosidase producing genes, essential for MOS degradation and present in the B. adolescentis 4-2 genome, were evaluated using enzymatic assays. Results: B. adolescentis 4-2 significantly increased fecal GABA levels within the KUHIMM. MOS supplementation elevated GABA levels, reduced fecal culture pH, and increased the relative abundance of Actinobacteria, particularly the Bifidobacterium species. Notably, the combined application of MOS and B. adolescentis 4-2 produced a synergistic increase in GABA production compared with the individual application of each component. β-mannosidase activity assays confirmed the effective utilization of MOS by B. adolescentis 4-2, supporting its role in enhancing GABA biosynthesis. Conclusion: These findings demonstrate a synergistic interaction between MOS and B. adolescentis 4-2 in promoting microbial GABA production in the KUHIMM in vitro. This synbiotic combination shows promise for modulating gut-derived GABA levels in vitro and warrants further investigation in animal models and human clinical studies.