Escherichia coli, one of the most common gut microbes, must survive through the highly acidic environment of the stomach at a mean of pH 2 before reaching the intestines. One method by which E. coli survive the acidity is by expelling protons from the cell, raising the intracellular pH to a more viable level, most commonly with the decarboxylation of glutamate into γ-aminobutyric acid (GABA) via glutamate decarboxylase. The GABA expelled from the cell contains more protons than the glutamate or glutamine pumped in which, in turn, raises intracellular pH. Because GABA functions as a neurotransmitter in humans, excess or decreased expression of glutamate decarboxylase may yield behavioral effects. We aimed to investigate GABA production in different strains of laboratory-evolved E. coli after two hours of acid stress at pH 2 in media with varying concentrations of either glutamine or glutamate to further understand what happens to GABA production in different conditions. Statistical analysis reveals the addition of glutamine to the media yields a similar concentration of extracellular GABA as the addition of glutamate does (p=0.6527). Interestingly, none of the laboratory-evolved E. coli strains produced more GABA than the ancestral strain. Four strains evolved in pH 4.6 media lost all GABA production, as did two strains evolved in pH 6.5 media with benzoate. Three other benzoate-evolved strains showed reduced production of extracellular GABA when compared to the ancestral strain (p<0.001). These results suggest variations in E. coli evolution under stress conditions may yield reduced production of GABA.
Camperchioli, Dominic W.; Bush, Sean P.; and Slonczewski, Joan, "GABA Production Through Acid Resistance Mechanisms in Escherichia coli" (2015). Kenyon Summer Science Scholars Program. Paper 5.