GABA-Producing Bacteria as Potential Psychobiotics in Gut–Brain Axis Regulation

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) and plays a vital role in maintaining neural balance, regulating mood, and reducing stress responses. Recent metagenomic studies of the gut microbiome have shown that various bacterial species, especially those in the genera Lactobacillus, Bifidobacterium, and Bacteroides, isolated from the human gut and environmental sources such as fermented foods, contain glutamate decarboxylase (GAD) systems that enable GABA production. Microbially produced GABA can influence the microbiota–gut–brain (MGB) axis by activating neural, endocrine, and immune signalling pathways that are crucial for maintaining gut and brain homeostasis. Emerging evidence suggests that supplementation with GABA-producing bacteria, known as psychobiotics, may improve neurotransmitter balance, modulate cytokine production, strengthen the integrity of the intestinal barrier, and alleviate anxiety- and depression-related behaviours. This review summarises current knowledge of GABA-producing bacterial strains derived from the human gut and food environments and explores their potential as emerging psychobiotics in modulating gut–brain communication and mental health.