Effect of Glutamate Concentration and Atmosphere of Incubation on the Production of ɣ-Aminobutyric Acid in Levilactobacillus brevis LB12

<i>Levilactobacillus brevis</i> is able to produce ɣ-aminobutyric acid (GABA), a non-proteogenic amino acid that provides several benefits to human health. In this study, we investigated the effect of glutamate (Glu) and oxygen (O₂) on biomass yield, GABA production and regulation of the <i>gad</i> operon in <i>Lvb. brevis</i> LB12. A change in incubation atmosphere from anaerobiosis (AN) to aerobiosis (AE) was applied to elucidate if AE pre-adaptation and cultivation could be exploited to improve cell density, as well as to determine the role of O₂ on the expression of the <i>gad</i> operon. AE increased biomass yield, but impaired Glu to GABA conversion, in both the cultivation and the adaptation phases. The <i>gad</i> operon (<i>gadR</i>, <i>gadC</i>, <i>gadB</i>, <i>gltX</i>) was up-regulated in the presence of Glu, while O₂ strongly reduced the transcription of <i>gadC</i> and <i>gadB</i>. Switching the incubation atmosphere (AE vs. AN) and Glu supplementation did not restore the gene functionality, suggesting that the negative effect of O₂ was persistent and more prolonged adaptation to AN would be required. This study provides additional data on the regulation of the <i>gad</i> operon, but further insight on the effect of O₂ upon GABA production by <i>Lvb. brevis</i> must be expanded to understand the possible mechanisms involved.