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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Siyun is 100% dedicated to the success of her students. She always encourages us to follow our own path and is happy to provide whatever support is needed to accomplish our goals. She is also committed to fostering a cooperative team atmosphere, which makes working in her research group a real pleasure. She is a major driving force behind many of my successes.
Numerous outbreaks in North America have been attributed to non-typhoidal Salmonella enterica. Bacteriophages (phages), viral bacterial predators, represent agents that could be used for controlling Salmonella; yet, relatively little is known about phages and their host interactions. Therefore, the purpose of this thesis was to characterize phages of Salmonella on their phenotypic and genomic determinants and phage-host interactions. Salmonella phages (n=97) were isolated from sites within British Columbia, Canada. Host range analysis revealed diverse patterns of lysis, with several broad host range phages identified. Initial screening demonstrated that phage SI1 exhibited remarkable stability at a variety of pH and temperature values. Artificially- contaminated sprouting alfalfa seeds treated with SI1 resulted in a significant (p98% nucleotide identity), yet between clusters, genomes exhibited a span of diversity (
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
Consumption of alfalfa sprouts has increased worldwide due to the nutritional value and health benefits of sprouts. However, alfalfa sprouts contaminated with Salmonella enterica (S. enterica) have been the source of many foodborne outbreaks in Europe and North America. Antimicrobial treatments for sprouted seeds are recommended by the Canadian Food Inspection Agency but the influence of different antimicrobial sanitation seed treatments on the behaviour of S. enterica during seed germination remains unclear. The goals of this study were (1) to investigate the ability of S. enterica to grow on sprouting alfalfa seeds after three different sanitation seed treatments to reduce microbial load, and (2) to understand how colonization by S. enterica and different antimicrobial treatments affect metabolites released by sprouting alfalfa seed.Alfalfa seeds inoculated with five strains of S. enterica were subject to three different seed treatments: (1) sodium hypochlorite (NaClO), (2) hydrogen peroxide (H₂O₂), and (3) an organic treatment involving a hot water dip, treatment with H₂O₂ and acetic acid. The disinfected seeds were sprouted to identify the growth characteristics of S. enterica after sanitation stress (n = 3). The populations of all five S. enterica strains which were present at
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.