Physiology of γ-aminobutyric acid production by .

Gut bacteria hold the potential to produce a broad range of metabolites that can modulate human functions, including molecules with neuroactive potential. One such molecule is γ-aminobutyric acid (GABA), the main inhibitory neurotransmitter of the central nervous system in animals. Metagenomic analyses suggest that the genomes of many gut bacteria encode glutamate decarboxylase (GAD), the enzyme that catalyzes GABA production. The genome of , a mucin specialist and potential next-generation probiotic from the human gut, is predicted to encode GAD, suggesting a contributing role in GABA production in the human gut. In this study, was grown in batch cultures with and without pH control. In both experiments, was found to produce GABA as a response to acid (pH <5.5), although only when GABA precursors, either glutamate or glutamine, were present in the medium. Proteomic analysis comparing grown with and without precursors at pH 4 did not show a difference in GAD expression, suggesting that it is expressed regardless of the presence of GABA precursors. To further investigate the function of GAD, we heterologously expressed the gene (encoded by locus tag Amuc_0372) with a His tag in and purified the GAD protein. Enzyme assays showed GAD activity in a pH range between 4 and 6, with the highest specific activity at pH 5 of 144 ± 16 µM GABA/min/mg. Overall, our results demonstrate the ability of to produce GABA as an acid response and unravel the conditions under which GABA production in occurs.IMPORTANCE is considered to be a beneficial bacterium from the human gut, but the exact mechanisms by which influences its host are not yet fully understood. To this end, it is important to identify which metabolites are produced and consumed by that may contribute to a healthy gut. In the present study, we demonstrate the ability of to produce γ-aminobutyric acid (GABA) when grown in an acidic environment, which often occurs in the gut. GABA is the major inhibitory neurotransmitter in the central nervous system and is present in the human gut. For this reason, it is considered an important bacterial metabolite. Our finding that produces GABA in acidic environments adds to the growing body of understanding of its relationship with host health and provides an explanation on how it can survive acid stress in the human gut.