Screening of glutamate decarboxylase activity and bile salt resistance of human asymptomatic carriage, clinical, food, and environmental isolates of Listeria monocytogenes

Following consumption, stomach acidity is the first major barrier encountered by the food-borne pathogen Listeria monocytogenes. Analysis of low pH sensitivity and glutamate decarboxylase (GAD) acid resistance system of 14 isolates of L. monocytogenes carried asymptomatically by humans showed that levels of GAD activity were subjected to strain variation. Similar variations were observed for strains responsible for 18 cases of listeriosis, whereas in comparison, 13 strains isolated from food and food-processing plant environments showed lower GAD activity. Following survival of the stomach barrier, L. monocytogenes also has to resist bile salts encountered in the small intestines. Analysis revealed that all strains tested were able to grow in the presence of bile salts with concentrations as high as those encountered in the small intestines and had previously identified bile salt hydrolase (BSH) activity. Strain variation was observed but there was no relationship between the origin of the strains and the ability to degrade bile salts.