Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2003
Publication Date: 2/1/2004
Citation: Bayles, D.O. 2004. Changes in thermal sensitivity resulting from ph and nutritional shifts of acid-adapted and non-acid-adapted listeria monocytogenes scott a, a serotype 4b strain. Journal of Food Protection. 67:316-321. Interpretive Summary: Listeria monocytogenes is the causative agent of listeriosis, a food-borne illness that has the highest case fatality rate of all bacterial food-borne infections. Sporadic and outbreak cases of listeriosis have been traced to a variety of foods, indicating that under some conditions, L. monocytogenes is able to survive in foods and cause disease. It is known that bacteria respond to their surroundings in an effort to survive and grow; however, it is not well understood how these adaptations affect our ability to destroy bacteria in foods during heating. It is also unclear how the food composition itself alters our ability to destroy bacteria that contaminate foods. To better understand these interactions, L. monocytogenes was provoked to develop an adaptive response (by exposure to acidic conditions), and both provoked and unprovoked bacteria were subsequently heat treated in a combination of media that simulated different food conditions. Analyses of the results uncovered the effects and interactions that nutrient availability, glucose (sugar) availability, and low or neutral pH had on the ability to destroy L. monocytogenes with heat. Information on these effects and interactions helps to reveal food conditions that may require additional interventions to assure safety and may also lead to changes that can be incorporated into foods that will improve the effectiveness of current interventions.
Technical Abstract: Stationary phase Listeria monocytogenes cells that were either pH-dependent acid-adapted or non-acid-adapted, were heat challenged at 60C in fresh and conditioned tryptic soy broth(TSB) media, with and without 1% glucose, at pH 4.8 and pH 7. Analysis of the D60-values indicated that non-acid-adapted cells were most thermally resistant when challenged in fresh medium (D60 = 1.16 min) and displayed the highest heat resistance when challenged in pH7 fresh medium containing 1% glucose (D60 = 1.52 min). The presence of glucose caused non-acid-adapted cells challenged at pH 7 to be more thermally resistant (D60 = 1.03 min) than when inactivated at pH 4.8 (D60 = 0.65 min) indicating that the presence of glucose is more important to thermal resistance in near neutral environments with available nutrients. Acid-adapted cells were the most thermally sensitive when challenged in pH 4.8 conditioned medium containing gglucose (D60 = 0.41 min) and were most thermally resistant when challenged in fresh medium without glucose at pH 4.8 (D60 = 0.91 min). The presence of glucose during the thermal challenge of acid-adapted cells significantly decreased the thermal resistance (D60 = 0.77 min without glucose, D60 = 0.59 min with glucose). The D60-values of acid-adapted cells challenged in pH 4.8 conditioned medium (D60 = 0.52 min) were significantly lower than values obtained when cells were challenged in pH 7 conditioned medium (D60 = 0.76min). The survivor data was modeled to extract information on the distribution of thermal sensitivity in the challenged populations. These data indicated that the presence and level of cross protection is highly dependent on the physiological state of the cells and nutrient availability at the time of thermal challenge.