Presence of GadD1 Glutamate Decarboxylase in Selected Listeria monocytogenes Strains Is Associated with an Ability To Grow at Low pH

Abstract

The glutamate decarboxylase (GAD) system is critical to the survival of Listeria monocytogenes LO28 at low-pH stress (<pH 4.0). The GAD system classically involves two proteins, a glutamate decarboxylase enzyme coupled to a glutamate/␥-aminobutyrate antiporter, which results in the consumption of an intracellular proton for each glutamate entering the system. Uniquely among prokaryotes, some strains of L. monocytogenes, including strain LO28, possess genes encoding three decarboxylases (gadD1, gadD2, and gadD3) and two antiporters (gadT1 and gadT2). These are organized in two pairs (gadD1T1 and gadD2T2) and a distinct gadD3. While the creation of a gadD3 mutant has not been possible, analysis of 15 isogenic mutants has confirmed previous observations that GadD2/T2 are primarily responsible for surviving severe acid challenge (pH 2.8). However, we have now established that GadD1 plays a major role in growth at mildly acidic pHs (pH 5.1). When strain variation studies revealed that a large number of L. monocytogenes strains (including all serotype 4 strains) lack the gadD1 gadT1 pair, low-pH growth assays were carried out. It was found that the majority of strains that grew poorly at pH 5.1 lacked these genes. The strain-variable ability to grow in mildly acidic conditions may explain why non-serotype 4 strains of L. monocytogenes predominate in foods.

Glutamate Decarboxylase-Mediated Nisin Resistance in Listeria monocytogenes

“…While the other loci that have been shown to contribute to nisin tolerance in L. monocytogenes are present in all Listeria strains, the gadD1T1 operon is unusual in that it is present on an islet of five genes that is absent from a large number of L. monocytogenes strains (8,23). We therefore hypothesized that the natural presence or absence of gadD1 may impact on the nisin tolerance of individual strains.…”

“…1). Mutation of specific gad genes renders cells exquisitely sensitive to ex vivo porcine and synthetic human gastric fluid and significantly impairs growth and survival in low-pH foods (4,7,8). Given the link between acid and nisin resistance phenotypes, the present study was initiated in order to investigate the contribution, if any, of gad genes to the nisin tolerance of L. monocytogenes.…”

Contributions of Listeria monocytogenes σ B and PrfA to expression of virulence and stress response genes during extra- and intracellular growth

“…While s B -dependent opuCA transcription under salt stress has been reported previously, the current study establishes reference values for transcript levels of s B -dependent genes in extracellular L. monocytogenes at 37 u C, which were needed to characterize relative s B activity in intracellular L. monocytogenes. As Milohanic et al (2003) showed that the s B -dependent opuCA also may be co-regulated by PrfA, we measured expression of a second s B -dependent gene, gadA, [lmo2434 (Conte et al, 2002); which is also designated gadD (Wemekamp-Kamphuis et al, 2004) and gadD3 (Cotter et al, 2005)]. s B activity (as determined by gadA and opuCA transcript levels) in intracellular L. monocytogenes arrested in either the host cell vacuole or the host cell cytosol was higher than in unstressed, exponentially growing bacteria and lower than in bacteria exposed to extracellular environmental stresses.…”

Lantibiotic Resistance

“…In L. monocytogenes, several systems help the organism to withstand low-pH stress, but the glutamate decarboxylase (GAD) system is probably the most important (197)(198)(199). Of the five gad genes, gadD1, gadD2, and gadD3 encode decarboxylases that catalyze the conversion of glutamate to ␥-aminobutyrate (GABA) and carbon dioxide (CO 2 ).…”