Characteristics of Listeria monocytogenes Important for Pulsed Electric Field Process Optimization
Processing-resistant Listeria monocytogenes strains appear sporadically in ready-to-eat food, and constitute a considerable challenge to food processors. Pulsed electric field (PEF) is an alternative to thermal processing and it inactivates pathogens in liquid food. The high-intensity electric pulses kill rapidly microbial contaminants. Since L. monocytogenes is among the most PEF-resistant non-sporing foodborne pathogens, this study focused on identifying characteristics of Listeria that may be useful for PEF process optimization. Resistance to PEF treatment, at 25 kV/cm, varied among L. monocytogenes strains suspended in 0.1% NaCl (pH 7.0) or 50% acid whey (pH 4.2). Listeria monocytogenes OSY-8578 was identified as a potential target strain for process optimization, based on its high PEF-resistance. Resistance to PEF could not be associated with genotypic differences, as determined by pulsed field gel electrophoresis or arbitrarily primed-polymerase chain reaction method. However, the target strain, OSY-8578, had a unique genotype, which is potentially useful for its identification. Sublethal PEF process (15 kV/cm for 29 micros) did not induce general stress response, a physiological state with over-expression of major molecular chaperones. On the contrary, expression of the chaperones, GroEL, GroES and DnaJ decreased within 5 to 20 min after PEF treatment. The decrease in expression was more marked for Scott A (PEF-sensitive) than OSY-8578 (PEF-resistant). Interestingly, the chaperone expression profile, after PEF treatment, paralleled an increased sensitivity of the pathogen to mild heat. This sensitization was not observed when mild heating was applied prior to PEF. Lactobacillus plantarum ATCC 8014, but not L. innocua ATCC 33090, was identified as potential surrogate of the target L. monocytogenes , OSY-8578, when these bacteria were suspended in a neutral simple medium (e.g., 0.1% NaCl solution) and treated at 25 kV/cm for 144 micros. Inactivation of Listeria by PEF was greater in acid or acidified whey (pH 4.2) than in sweet whey (pH 6.8) or in 0.1% NaCl (pH 7.0). In acid or acidified cheese whey, Lb. plantarum was more resistant to PEF than was L. monocytogenes OSY-8578; the former bacterium, therefore, may serve as a surrogate of the latter in such media.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:listeria pulsed electric field target chaperone groel groes dnaj surrogate sensitization
Date of Publication:01/01/2003