The glutamate decarboxylase system (GAD2) is activated by osmotic stress in a GadR-dependent manner in Listeria monocytogenes

Under acidic conditions, the glutamate decarboxylase system GAD2 protects Listeria monocytogenes by consuming intracellular protons. GAD2 is encoded by the gadT2D2 operon and regulated by the transcriptional regulator GadR. In this study we investigated whether osmotic stress, which is commonly used as a hurdle in food preservation, influences GadR activity. Exposure to 0.5 M or 1 M NaCl rapidly increased gadT2 transcription in a GadR-dependent manner. This induction was independent of Na+ or Cl- ions as other osmotically active molecules (e.g. KCl, Na2SO4, sucrose) produced similar effects, suggesting that this regulation is responsive to changes in osmolarity. Two putative GadR binding sites in PgadT2 were indispensable for this osmotic induction. Furthermore, GadR-dependent GAD2 expression is elevated in NaCl-treated cultures and it confers protection against lethal acid stress. Taken together these data showed that osmotic stress induces GadR-dependent GAD2 expression, which confers a significant survival advantage when cells are subsequently exposed to acid stress, likely due to an improved capacity for intracellular pH homeostasis. Thus, GadR is activated by two conditions commonly used in food preservation, mild acid pH and elevated osmolarity, which may increase bacterial resistance to gastric acid, posing a potential risk to food safety.