Survival of Listeria monocytogenes grown at 5, 10, and 37 degrees C in a simulated dynamic gastric model

The purpose of this study was to assess impact of previous growth temperatures and on survival during gastric transit on Listeria monocytogenes. L. monocytogenes strain ATCC 19115 was grown at 5, 10, and 37°C in tryptic soy broth with 0.6% yeast extract until desired turbidity was reached. Turbid broth with culture (unwashed) and pelleted cells by centrifugation and resuspension in phosphate buffer saline (washed) were inoculated into a simulated dynamic gastric model kept at 37°C. Initial population size was approximately 107 CFU/ml. Simulated gastric fluid was constantly pumped and the pH was corrected using 1N HCl to mimic conditions found in human stomach. Samples were taken at 15 minute intervals starting from 0 minute up to 120 minutes. Appropriate dilutions in duplicates were spiral plated onto modified oxford agar for enumeration of L. monocytogenes.; Irrespective of the growth temperature and cell preparation, L. monocytogenes survived the gradual decline of gastric pH from 5 to approximately 2 with less than 1 log10 CFU/ml decrease over 60 minutes. After 120 minutes of gastric exposure, both washed and unwashed cultures grown at 5 and 10°C were not detectable. Population decrease was more than 5 log10 CFU/ml. This same fate was seen for 37°C grown, washed cell population, whereas unwashed cells were still detectable after 120 minutes, with population decrease of 4.59 log10 CFU/ml. Survival rate for unwashed cells previously grown at 37°C was significantly greater in the second hour of gastric exposure than other treatments. Previous growth temperature history had little impact on survival of L. monocytogenes during gastric transit.