Doctor of Philosophy in Food Science (PhD)
The effect of native microflora on the growth of Listeria monocytogenes in soft-ripened raw milk cheeses
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Novel physiological and genetic factors associated with the survival of Listeria monocytogenes in the food-processing continuum were investigated, with an emphasis on its cold-growth ability. Food-related L. monocytogenes strains (n=166) were sequenced and subsequently evaluated on their ability to tolerate cold (4°C), salt (6% NaCl, 25°C), acid (pH 5, 25°C), and desiccation (33% RH, 20°C) stress. Stress tolerances were associated with serotype, clonal complex, full-length inlA profiles, and plasmid harbourage. Notably, strains possessing full length inlA (as opposed to a truncated version) exhibited significantly (p1,000-fold) of differentially expressed (e.g., >2-fold, p
Cucumbers have been associated with recent Salmonella enterica (S.enterica) outbreaks. The ability of S. enterica to attach or internalize into produce may be a factor that make these produce items more likely to be sources of S. enterica contamination. The purpose of this study was to evaluate the survival capability of S. enterica on mini cucumbers and explore the factors contributing to the survival of this foodborne pathogen on the surface of cucumbers.Five strains of S. enterica representing different serotypes were individually inoculated onto mini cucumbers and subsequently incubated at 22 ± 2 °C for 8 days or at 4 ± 2 °C for 19 days respectively. Crystal violet assay was performed to quantify the biofilm formation and attachment capability based on the value of optical density at 595 nm of the destaining crystal violet at the specific interval time (0, 3, 6, 9, 12, 24, 48, 72, 96 hours). The phenotypic evaluation of red dry and rough (rdar) morphotype formation of S. enterica were conducted on Luria-Bertani (LB) agar complemented with Congo red (40 μg/mL) and Coomassie brilliant blue (20 μg/mL). The results suggested different S. enterica strains showed differential survival rates at both temperatures. S. Poona exhibited the strongest survival ability at 22 ± 2 °C with the highest Δlog CFU and maximum achieved density (Nmax) of 0.84 ± 0.01 and 6.72 ± 0.05, respectively. However, at 4 ± 2 °C, S. Enteritidis survived better compared with S. Poona due to the least cell density decrease of -0.91 ± 0.01 Δlog CFU and maximum achieved density of 6.04 ± 0.09. Besides, survival behaviors of S. enterica were found to be associated with biofilm formation ability and the biofilm ability differed among different strains. This means that biofilm formation contributes to the survival ability of S. enterica on mini cucumbers. Lastly, different strains exhibited specific morphotypes on Congo red agar, indicating that both curli and cellulose contribute to biofilm formation of S. enterica. Unique survival characteristics among S. enterica reveal that corresponding interventions need to be applied to eliminate contamination of produce with specific S. enterica strains.