CHANGES IN BAROTOLERANCE, THERMOTOLERANCE AND CELLULAR MORPHOLOGY OF LISTERIA MONOCYTOGENES THROUGHOUT THE LIFE CYCLE
Open Access
- Author:
- Wen, Jia
- Graduate Program:
- Food Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- August 29, 2008
- Committee Members:
- Ramaswamy C Anantheswaran, Thesis Advisor/Co-Advisor
Stephen John Knabel, Thesis Advisor/Co-Advisor
Ramaswamy C Anantheswaran, Thesis Advisor/Co-Advisor - Keywords:
- life cycle
morphology
barotolerance
Listeria monocytogenes
thermotolerance - Abstract:
- Listeria monocytogenes is a psychrotrophic pathogenic bacterium that can grow in foods and cause a serious disease, listeriosis. It can be present in raw or Ready-To-Eat (RTE) foods and is one of the most pressure- and heat-resistant vegetative foodborne pathogens. High Pressure Processing (HPP) is a non-thermal process of increasing interest due to the needs for minimally processed foods in the market. HPP allows microbial inactivation while maintaining the fresh flavors of foods; however, non-linear inactivation (tailing) during HPP of L. monocytogenes, which is a potential danger to food safety, has been reported. My initial experiments focusing on the mechanism(s) producing tailing were unsuccessful. Therefore, the focus of this research turned to changes in barotolerance, thermotolerance and cellular morphology of L. monocytogenes throughout its life cycle. In part one, L. monocytogenes was grown to log, stationary, death and long-term survival phases at 35°C in tryptic soy broth with yeast extract (TSBYE). Cells were diluted in UHT whole milk and then high pressure processed at 400 MPa for 180 s or thermally processed at 62.8°C for 30 s. As cells transitioned from log to long-term survival phases, D400 MPa and D62.8°C increased 10 and 19 fold, respectively. Cells decreased in size as they transitioned from log to long-term survival phases, and rod-shaped cells transitioned to cocci as they entered late-death and long-term survival phases. In part two, cells in long-term survival phase were centrifuged, suspended in fresh TSBYE and incubated at 35°C. As cells transitioned from long-term survival to log and stationary phases, they increased in size and log reductions increased following HPP or heat treatment. In part three, cells in long-term survival phase were centrifuged, suspended in UHT whole milk and incubated at 4°C. After HPP or heat treatment similar results were observed as in part two. These results demonstrate that cells of L. monocytogenes are most barotolerant and thermotolerant in long-term survival phase, which needs to be considered when developing food processing or preservation methods to eliminate L. monocytogenes.