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The glutamate decarboxylase (GAD) system has been shown to be important for the survival of  Listeria monocytogenes  in low pH environments. The bacterium can use this faculty to maintain pH homeostasis under acidic conditions. The accepted model for the GAD system proposes that the antiport of glutamate into the bacterial cell in exchange for γ-aminobutyric acid (GABA) is coupled to an intracellular decarboxylation reaction of glutamate into GABA that consumes protons and therefore facilitates pH homeostasis. Most strains of  L. monocytogenes  possess three decarboxylase genes ( gadD1, D2  &  D3 ) and two antiporter genes ( gadT1  &  gadT2 ). Here, we confirm that the  gadD3  encodes a glutamate decarboxylase dedicated to the intracellular GAD system (GAD i ), which produces GABA from cytoplasmic glutamate in the absence of antiport activity. We also compare the functionality of the GAD system between two commonly studied reference strains, EGD-e and 10403S with differences in terms of acid resistance. Through functional genomics we show that EGD-e is unable to export GABA and relies exclusively in the GAD i  system, which is driven primarily by GadD3 in this strain. In contrast 10403S relies upon GadD2 to maintain both an intracellular and extracellular GAD system (GAD i /GAD e ). Through experiments with a murinised variant of EGD-e (EGDm) in mice, we found that the GAD system plays a significant role in the overall virulence of this strain. Double mutants lacking either  gadD1D3  or  gadD2D3  of the GAD system displayed reduced acid tolerance and were significantly affected in their ability to cause infection following oral inoculation. Since EGDm exploits GAD i  but not GAD e  the results indicate that the GAD i  system makes a contribution to virulence within the mouse. Furthermore, we also provide evidence that there might be a separate line of evolution in the GAD system between two commonly used reference strains.

plos one

Divergent Evolution of the Activity and Regulation of the Glutamate Decarboxylase Systems in Listeria monocytogenes EGD-e and 10403S: Roles in Virulence and Acid Tolerance