FIG. 1.

An overview of the L. monocytogenes glutamate decarboxylase (GAD) system. L. monocytogenes possesses five gad genes. gadD1, gadD2, and gadD3 encode decarboxylases which catalyze the conversion of glutamate to γ-amino butyrate (GABA) and carbon dioxide (CO₂). gadT1 and gadT2 encode glutamate-GABA antiporters. Nisin functions by binding to lipid II, an essential precursor of cell wall peptidoglycan synthesis. Binding to lipid II facilitates the formation of pores within the cytoplasmic membrane leading to the release of ATP and may ultimately result in cell death. We suggest that under certain conditions gadD1 may contribute to intracellular ATP pools and hence tolerance of nisin.