29G-1 |
Acid tolerance of Listeria monocytogenes as affected by environmental stresses related to food processing technologies |
K. Koutsoumanis1, P. A. Kendall2, and J. N. SOFOS1. (1) Department of Animal Sciences, Colorado State University, Fort Collins, CO 80523-1171, (2) Department of Food Science & Human Nutrition, Colorado State University, 1571 Campus Delivery, Fort Collins, CO 80523-1571 The bacterial "stress hardening" phenomenon has been postulated as a factor that may influence the safety of foods. Most of the available data on adaptive responses to stresses, refer to the effect of a single stress. During food processing, however, bacteria can be exposed to more than one stress at the same time. The objective of the present study was to evaluate the effect of single or combined environmental stresses, relevant to food processing technologies, on the acid resistance of Listeria monocytogenes. Stationary phase cells of L. monocytogenes, grown in glucose-free media, were exposed for 90 min to different stresses including pH (4.0-7.0), osmotic shock (10-20% NaCl), heat (45-50oC) or low temperature shock (-5oC) and further exposed to pH 3.5. Exposure to a mildly acidic (pH 5.0-6.0) environment provided protection of the pathogen against acid. This adaptive response, however, was found to be strongly dependent on other environmental conditions during the shock such as temperature or the presence of a second stress factor (NaCl). Growth of L. monocytogenes in the presence of glucose resulted in an enhanced survival of the pathogen at pH 3.5. The results indicated a difference in mechanisms of acid resistance between acid shocked (exposed to low pH) and acid-adapted (grown in the presence of glucose) cells. Sublethal stresses, other than acid, such as osmotic (10% NaCl), heat (45oC) and low temperature (-5oC), did not significantly affect the acid resistance of L. monocytogenes. More severe levels of these stresses (20% NaCl and 50oC), however, resulted in sensitization of the pathogen to acid. Since the above stress conditions tested in the present study are common within current food processing technologies, the data on adaptive responses of L. monocytogenes provided could lead to more realistic estimations of risks associated with consumption of processed foods and to processing modifications that could enhance food safety.
Session 29G, Food Microbiology: General
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