Studies on the Survival of Listeria monocytogenes in Filtered Fluid Milk Products Under Simulated Human Gastric Conditions (SHIME®) and on the Viable but Non-culturable State of the Organism
Listeria monocytogenes is an opportunistic foodborne pathogen which has been implicated in many outbreaks of foodborne diseases. The thesis examined the survival of L. monocytogenes under acid and chlorine stresses which it encounters inside the host or in the food processing environment, respectively. The Simulator of the Human Intestinal Microbial System (SHIME) was used to simulate the gastric conditions of human adults. The survival rates of L. monocytogenes inoculated in filtered milk products after a 2-hour exposure to simulated gastric fluid with pH values of 1.5, 2.0 and 3.0 were 0.003 to 0.040%, 22.7 to 43.4% and 16.6 to 27.2%, respectively. The survival of L. monocytogenes depended on a combination of factors, including gastric acidity, gastric digestion time, L. monocytogenes strain, food type and recovery method. The exposure of L. monocytogenes to 3.5 ppm chlorine induced the cells to enter the viable but non-culturable state (VBNC). The cells in the VBNC state showed signs of resuscitation and growth after 24 hours of incubation in filtered milk products at 4˚C.