Functional γ-Aminobutyrate Shunt in Listeria monocytogenes: Role in Acid Tolerance and Succinate Biosynthesis

Abstract

Listeria monocytogenes, the causative agent of human listeriosis, is known for its ability to withstand severe environmental stresses. The glutamate decarboxylase (GAD) system is one of the principal systems utilized by the bacterium to cope with acid stress, a reaction that produces ␥-aminobutyrate (GABA) from glutamate. Recently, we have shown that GABA can accumulate intracellularly under acidic conditions, even under conditions where no extracellular glutamate-GABA exchange is detectable. The GABA shunt, a pathway that metabolizes GABA to succinate, has been described for several other bacterial genera, and the present study sought to determine whether L. monocytogenes has this metabolic capacity, which, if present, could provide a possible route for succinate biosynthesis in L. monocytogenes. Using crude protein extracts from L. monocytogenes EGD-e, we show that this strain exhibits activity for the two main enzyme reactions in the GABA shunt, GABA aminotransferase (GABA-AT) and succinic semialdehyde dehydrogenase (SSDH). Two genes were identified as candidates for encoding these enzyme activities, argD (GABA-AT) and lmo0913 (SSDH). Crude protein extracts prepared from a mutant lacking a functional argD gene significantly reduced GABA-AT activity, while an lmo0913 mutant lost all detectable SSDH activity. The deletion of lmo0913 increased the acid tolerance of EGD-e and showed an increased accumulation of intracellular GABA, suggesting that this pathway plays a significant role in the survival of this pathogen under acidic conditions. This is the first report of such a pathway in the genus Listeria, which highlights an important link between metabolism and acid tolerance and also presents a possible compensatory pathway to partially overcome the incomplete tricarboxylic acid cycle of Listeria.

Mycobacterium tuberculosis Proteome Response to Antituberculosis Compounds Reveals Metabolic “Escape” Pathways That Prolong Bacterial Survival

“…GabT is one of the key enzymes of the glutamate decarboxylase (GAD) system that catalyzes glutamate to the ␥-aminobutyrate (GABA). The GABA shunt pathway has been suggested to provide an alternative source of succinate in bacteria and a way to generate energy from glutamate without production of ammonia (16,17). The in vitro and macrophage assays demonstrated that while delayed killing was observed for an M. smegmatis clone overexpressing the GabT protein during treatment with anti-TB drugs, significant reduction in time was seen in an M. tuberculosis GabT-deficient mutant.…”

Bacillus cereus cell response upon exposure to acid environment: toward the identification of potential biomarkers

“…Arginine decarboxylase enzyme has one substrate (L-arginine) and two products, CO2 and agmatine, which has been showed to be a competitive inhibitor in E. coli (Blethen et al, 1968). GAD converts glutamate in γ -aminobutyrate (GABA) and is well described in L. monocytogenes for instance (Cotter et al, 2001; Gandhi and Chikindas, 2007; Karatzas et al, 2010, 2012; Feehily et al, 2013). …”

Ultrasound processing of liquid system(s) and its antimicrobial mechanism of action

“…Similarly deletion of gabT and gabD that encode for the GABA shunt that catabolize intracellular GABA pools produced by the intracellular GAD system (Feehily et al . ), resulted in resistance in sterile water but not in wastewater. It has been suggested that the GAD system, together with the GABA shunt feed in the TCA cycle, are affecting the levels of succinate and oxoglutarate that have antioxidant properties and, thus can confer resistance to oxidant species (Ramond et al .…”