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SURVIVAL AND GROWTH OF LISTERIA MONOCYTOGENES IN CHOPPED GREEN AND RED BELL PEPPERS USING PREDICTIVE MODELING
Title
SURVIVAL AND GROWTH OF LISTERIA MONOCYTOGENES IN CHOPPED GREEN AND RED BELL PEPPERS USING PREDICTIVE MODELING
Creator
Zhang, L1jie
Date
2015, 2015-07
Description
Listeria monocytogenes (L. monocytogenes) is a Gram-positive pathogenic organism and the causative agent of human and animal listeriosis....
Show moreListeria monocytogenes (L. monocytogenes) is a Gram-positive pathogenic organism and the causative agent of human and animal listeriosis. Listeriosis is a gastrointestinal or invasive systemic illness resulting from consumption of contaminated food products, mainly cheese, deli meats, and fresh produce, by L. monocytogenes. In recent years, several foodborne outbreaks have been reported that were associated with fresh produce, such as cantaloupe, celery and sprouts. Temperature is considered a major factor that affects L. monocytogenes growth during storage. The proliferation of L. monocytogenes varies on different produce items based on storage temperature. In this study, the persistence and population dynamics of three L. monocytogenes strains, LS806 (cheese isolate), LS810 (cantaloupe isolate) and LS808 (celery isolate) were evaluated by incubating inoculated fresh-cut green bell pepper and red bell pepper at various temperatures (5oC, 10oC, and 25oC) for 14 days. To assess the risk of L. monocytogenes in these fresh-cut vegetable items, a primary predictive model was fitted for L. monocytogenes growth data using DMFit. Green bell pepper had significantly (P<0.05) higher pH and aw, and higher amounts of yeast and mold and Enterobacteriaceae than did red bell pepper. In green bell pepper, all three strains showed no significant difference (P>0.05) in growth rate when incubated at the same temperature. In red bell peppers, LS808 had the highest (P<0.05) growth rate at both 5°C and 25°C out of the three strains. All of the three strains grew significantly faster (P<0.01) at 25°C than either 5°C or 10°C in both green and red bell peppers. All three strains obtained less than 1 log10 growth increase after incubating at 25°C for 6 hours on pre-chilled produce. Some strains (LS806 and LS810) significantly increased (P<0.01) during two 5-hour 25°C incubations, but did not reach 1 log10 growth increase. The results indicate that L. monocytogenes not only persists, but also grows in chopped green and red bell peppers at 5, 10, and 25°C, and strains grew faster at the higher temperature (25°C). Data obtained could be further evaluated for determining whether Time/temperature control for safety (TCS) designation should be applied to chopped green and red bell peppers.
M.S. in Food Safety and Technology, July 2015
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THE TRANSCRIPTIONAL RESPONSE OF ARTIFICIALLY-INOCULATED LISTERIA MONOCYTOGENES DURING THE MANUFACTURE OF UNPASTEURIZED GOUDA CHEESE
Title
THE TRANSCRIPTIONAL RESPONSE OF ARTIFICIALLY-INOCULATED LISTERIA MONOCYTOGENES DURING THE MANUFACTURE OF UNPASTEURIZED GOUDA CHEESE
Creator
Carstens, Christina K.
Date
2017, 2017-05
Description
Listeriosis outbreaks indicate that L. monocytogenes contamination is an issue for various food types, such as unpasteurized (raw) cheese....
Show moreListeriosis outbreaks indicate that L. monocytogenes contamination is an issue for various food types, such as unpasteurized (raw) cheese. Current regulation prevents the interstate sale and distribution of raw cheese and mandates a ≥60 day aging period at ≥2˚C to ensure product safety; yet studies demonstrate that pathogens can persist during aging. Environmental stress can alter the transcriptomic profiles of pathogens; however, these surveys are rarely conducted in food matrices. This study aimed to assess the transcriptomic profiles of L. monocytogenes strain F2365 during environmental stressors inherent throughout cheesemaking. First, quantitative polymerase chain reaction (qPCR) was used to monitor transcription levels of nine L. monocytogenes genes involved in virulence, stress response, energy transport, and metabolism during osmotic stress (11% NaCl) and varying temperature/time (5˚C 24 h, 25˚C 24 h, 38˚C 30 min) conditions in Brain Heart Infusion (BHI) broth, raw milk, and pasteurized milk. Generally, the genes prfA, lmo1381, lmo0963, and lmo1875 were down-regulated, whereas the genes lmo1864, lmo0914, lmo0348, lmo1428, and lmo1264 were up-regulated. Virulence gene, prfA, was most down-regulated when L. monocytogenes was grown in raw milk with salt at 25˚C (4774.72±838.14 fold; relative to 24 h growth at 37˚C). The stress response gene lmo0914, encoding σB, was most up-regulated when L. monocytogenes was grown in BHI at 25˚C with salt (14.61±7.72 fold). Additionally, transcription levels of the nine genes were assessed at points during the laboratory-scale manufacture of Gouda cheese made with raw milk artificially-inoculated with L. monocytogenes via qPCR. Similar differential regulation for both prfA and lmo0914 in L. monocytogenes was observed during cheesemaking. The gene lmo1864, encoding a putative pore-forming hemolysin, was up-regulated throughout the cheesemaking process, but was most up-regulated after stirring the curd (449.81±432.53 fold). Ultimately, these results indicate that the lmo1864 gene may play a role in L. monocytogenes survival during cheesemaking. Methods developed in this study can be used to assess the risk of L. monocytogenes, not only during cheesemaking, but during the ≥60-day aging process. Overall, these results contribute to the understanding of L. monocytogenes survival mechanisms during the cheesemaking process.
M.S. in Food Safety and Technology, May 2017
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