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Abstract
BackgroundIn lactic acid bacteria (LAB), acid stress leads to decreases of cell vitality and fermentation yield. Glutamate decarboxylase (GAD) system is regarded as one of the essential acid-resistance mechanisms in LAB. However, the regulation of GAD system is not well identified in the genus Lactobacillus. Although potential transcriptional regulator gene located upstream of GAD system genes was found in several Lactobacillus species, such as Lactobacillus (L.) brevis, the contribution of the regulator to acid resistance of the genus Lactobacillus has not been experimentally determined.ResultsThe potential transcriptional regulator gene gadR was disrupted by homologous recombination in L. brevis ATCC 367, leading to the decreased expression of gadC and gadB. The inactivation of GadR completely eliminated γ-aminobutyric acid (GABA) production and decreased the glutamate-dependent acid resistance. Moreover, expression of gadC and gadB in the presence of glutamate was increased and glutamate also stimulated the expression of gadR. In addition, L. brevis D17, a strain screened from acidic fermented grains of Chinese liquor production, had much higher expression level of gadR than the typical strain L. brevis ATCC 367. Under the pH-controlled and mixed-feed fermentation, L. brevis D17 achieved a titer of 177.74 g/L and a productivity of 4.94 g/L/h of GABA within 36 h. However, the L. brevis ATCC 367 only achieved a titer of 6.44 g/L and 0.18 g/L/h of GABA although the same fermentation control approach was employed.ConclusionsGadR is a positive transcriptional regulator controlling GABA conversion and acid resistance in L. brevis. L. brevis strains with hyper-expressing of gadR are excellent candidates for GABA production in industrial scale.
Highlights
In lactic acid bacteria (LAB), acid stress leads to decreases of cell vitality and fermentation yield
Disruption of gadR eliminates GadR completely eliminated γ-aminobutyric acid (GABA) production Based on the genomic context analysis, the gene organization of gadR-gadC-gadB in L. brevis genome showed the same organization like that in Lc. lactis [8, 10]
These results showed that gadR was successfully disrupted in strain 367ΔgadR
Summary
In lactic acid bacteria (LAB), acid stress leads to decreases of cell vitality and fermentation yield. Glutamate decarboxylase (GAD) system is regarded as one of the essential acid-resistance mechanisms in LAB. LAB employ various types of acid resistance mechanisms to counteract the acidic stress, including the F1-F0-ATPase proton pump, the glutamate decarboxylase (GAD) system, the alkali production pathways, the formation of exopolysaccharides (dextran, reuteran, and levan), and repairing macromolecules [6, 7]. Among these mechanisms, the GAD system is regarded as one of the essential acid resistance mechanisms in. Many researchers focus on isolating the GABA hyper-producing strains, optimizing the medium composition and fermentation condition for GABA production, characterizing GAD, and increasing the activity of GAD by genetic modification [16, 18, 19]
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