Gamma-aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in the central nervous system (CNS). It is biosynthesized by glutamate decarboxylase, which catalyzes the alpha-decarboxylation of glutamate to GABA and CO2. Glutamate is an excitatory neurotransmitter in the CNS. Hence, the balance between these two neurotransmitters is crucially important to maintain a stable nervous condition. GABA production has been reported in several species of gut microbiome belonging to the families Bifidobacteriaceae, Lactobacillaceae, Bacteroidaceae, Enterococcaceae, Propionibacteriaceae, and Streptococcaceae. Bifidobacterium exhibits the ability to produce GABA from specific strains belonging to the species B. dentium, B. angulatum, B. adolescentis, and B. longum subsp. infantis.
GABA-producing bacteria are considered glutamate consumers as glutamate activates enzymatic conversion using microbial glutamate decarboxylases. Microbial GABA can pass from the gut to other organs through several pathways, including the blood or vagal pathways. It has been reported that mental disorders, such as depression, are negatively correlated with the abundance of GABA-producing Bacteroides. Further, accumulating evidence suggests that the ingestion of GABA-producing bacteria (probiotics) supports relief from psychiatric illnesses, such as depression, and physical ailments, such as diabetes.
Understanding the relationship between microbial composition and the level of fecal neurotransmitters can highlight the vital role of some microbes which can redirect the microbiome activity toward GABA production.
Recent studies support the important role of GABA in irritable bowel syndrome (IBS). High GABA concentrations relieve abdominal pain while low GABA concentrations are associated with increased abdominal tenderness and pain in patients with IBS. In these cases, the action of GABA is achieved through local receptors in the intestine but also by action in the CNS through the gut-brain axis.