33-11

A. TODOROVIC and D. L. Marshall. Dept. of Food Science, Nutrition & Health Promotion, Mississippi State Univ., Mailstop 9805, Mississippi State, MS 39762-9805

Most naturally occurring outbreaks of listeriosis involve growth of the bacterium in some foods at ambient or refrigerated temperatures rather than at the temperature 37ºC commonly used in research on pathogenesis and host defense. Also, the influence of gastric fluid in predictive studies on L. monocytogenes is underestimated due to use of static gastric model, without simulating the sequential pH change. The objective 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 TSB with 0.6% yeast extract. Washed and unwashed cells 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. Samples were taken at 15 minute intervals starting from 0 minute up to 120 minutes. Appropriate dilutions were plated onto selective agar, incubated and enumerated. 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. This study stresses the importance of applying dynamic gastric model rather than static one (with constant pH). The results suggest that 37°C growth temperature is not suitable for studies of pathogenesis of L. monocytogenes.

Session 33, Food Microbiology: General
9:00 AM - 12:00 PM, Monday AM Room 395

2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana