The glutamate decarboxylase (GAD) acid resistance system of the foodborne pathogen Listeria monocytogenes plays a major role in its survival at low pH. It was found that survival of the wild-type strain. LO28, in acidified reconstituted skim milk. diluted to reduce free glutamate levels. improves in response to supplementation with monosodium glutamate. A mutant, in which the two listerial GAD homologs have been deleted (and in which there is no discernible GAD activity), did not respond to glutamate supplementation and displayed greatly enhanced sensitivity in a number of low pH foods, even when levels of free glutamate were as low as 0.22 mM. We thus show that the GAD system plays a major role in the survival of L. monocytogenes in acidic foods even when levels of free glutamate are low.