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STUDY OF SALMONELLA SURVIVAL ON THE SURFACE OF FRUITS
Title
STUDY OF SALMONELLA SURVIVAL ON THE SURFACE OF FRUITS
Creator
Sui, Ke
Date
2017, 2017-05
Description
Salmonella enterica has been frequently linked to the major foodborne disease outbreaks. The molecular mechanisms underlying this pathogen...
Show moreSalmonella enterica has been frequently linked to the major foodborne disease outbreaks. The molecular mechanisms underlying this pathogen survival on the fresh fruit surface remain largely unexplored. In this study, the environmental factors that affect the survival of Salmonella strains on the surface of selected fruits were studied. Grape tomatoes (or cantaloupe peels) were inoculated in three separate trials with 1 mL Salmonella enterica serotypes Enteritidis or Typhimurium (approximately 1010-11 CFU/mL). Storage of grape tomatoes at 4 ℃ resulted in significant decrease in populations of S. Enteritidis; this trend was observed at both of the tested relative humidity with the D-value as 7.25±1.05 d and 7.28±2.34 d, respectively. At different temperatures, relative humidity only had marginal effects on the bacterial survival on the surface of grape tomato and cantaloupe. In addition, S. Typhimurium apparently survived better than S. Enteritidis on the surface of grape tomato. Furthermore, a transposonmutagenesis library with random transposon insertion mutants of S. Enteritidis and high-throughput sequencing analysis showed that the expression of genes rcsB and nlpD were hypothesized to be associated with the survival of S. Enteritidis on grape tomatoes. Inframe deletions of the two genes in S. Enteritidis were constructed by lambda red recombinase system and respective complementation mutants were also obtained for identification of the contribution of the two genes to the bacterial survival on the surface of grape tomato. Thus, this study provided microbiological and molecular microbiological basis to probe the roles of putative genes in Salmonella enterica survival on the surface of fresh fruits.
M.S. in Food Safety and Technology, May 2017
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SURVIVAL AND ATTACHMENT OF SALMONELLA ENTERICA ON ALMOND, HAZELNUT, AND BLACK PEPPER
Title
SURVIVAL AND ATTACHMENT OF SALMONELLA ENTERICA ON ALMOND, HAZELNUT, AND BLACK PEPPER
Creator
Zhai, Yang
Date
2016, 2016-05
Description
S. Enteritidis (SE) and S. Typhimurium (ST) are the two most common Salmonella enterica serovars that cause foodborne illnesses in U.S....
Show moreS. Enteritidis (SE) and S. Typhimurium (ST) are the two most common Salmonella enterica serovars that cause foodborne illnesses in U.S. Outbreaks of these serovars have been attributed to low moisture foods such as nuts and spices. However, little is known about the mechanisms of attachment on low moisture food surfaces. This study evaluated the associations between attachment and survival of Salmonella enterica serovars, storage temperatures, and surface characteristics of selected food samples. In the study, S. Enteritidis PT4 and S. Typhimurium LT2 were inoculated onto the surfaces of black peppercorns, almonds and hazelnuts at ~108 CFU/g. After 2 h air-drying, samples were stored at 4 and 25°C with 58 ± 2% relative humidity for up to 14 d. At specific time points, the food sample were washed two times in 1 × Phosphate Buffered Saline (PBS) to remove attached bacterial cells. The bacterial suspensions were serially diluted and plated on Brain Heart Infusion (BHI) and Xylose Lysine Deoxycholate (XLD) agar plates for enumeration. Significant differences (P<0.05) were found in the survival and attachment of SE and ST under all the conditions. ST can survive and attach better than SE at both 4 and 25°C. The decline rates on almonds, hazelnuts, and black peppercorns at 25°C were 0.348 ± 0.017, 0.273 ± 0.015, and 0.196 ± 0.017 log CFU/g per day for SE and 0.077 ± 0.008, 0.157 ± 0.008, and 0.048 ± 0.005 log CFU/g per day for ST, respectively. The attachment rates on almonds, hazelnuts and black peppercorns at 25°C were 18.71 ± 14.38, 1.56 ± 1.50, and 4.68 ± 0.76% for SE and 38.46 ± 11.32, 18.45 ± 7.73, and 56.30 ± 14.72% for ST, respectively. In addition, low temperature (i.e. 4°C) may contribute more to the survival of Salmonella than ambient temperature (i.e. 25°C). Certain surface characteristics (roughness & hydrophobicity) of hazelnut may probably result in weaker attachment of both Salmonella serovars. The results can be used to better understand the physiology of Salmonella enterica on low moisture foods and aid in developing effective control measures to reduce pathogen contamination.
M.S. in Food Safety and Technology, May 2016
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GROWTH AND SURVIVAL OF SALMONELLA IN PARTIALLY SPROUTED PUMPKIN, SUNFLOWER, AND CHIA SEEDS SUBSEQUENTLY DRIED FOR DIRECT CONSUMPTION
Title
GROWTH AND SURVIVAL OF SALMONELLA IN PARTIALLY SPROUTED PUMPKIN, SUNFLOWER, AND CHIA SEEDS SUBSEQUENTLY DRIED FOR DIRECT CONSUMPTION
Creator
Wang, Can
Date
2017, 2017-05
Description
Products based on dried, partially sprouted seeds have been associated with foodborne salmonellosis. Typically, sprouted seeds have been a...
Show moreProducts based on dried, partially sprouted seeds have been associated with foodborne salmonellosis. Typically, sprouted seeds have been a major concern with respect to foodborne illness, but little is known about the risks associated with dried partial sprouted seed products. Previous studies suggest that low-temperature, highhumidity storage conditions may provide an alternative means to inactivate Salmonella for those processors opposed to traditional thermal processing. The objective of this study was to determine the growth of Salmonella during germination of pumpkin, sunflower, and chia seeds, and the survival of Salmonella during drying and storage under variable temperature and relative humidity (RH) conditions. Pumpkin, sunflower, and chia seeds were dry inoculated at 5 log CFU/g with 4 serotype mixture of Salmonella using a dry transfer method, and then soaked in sterile water at 25 and 37°C for 24 h. Salmonella and total aerobic mesophilic plate counts (APC) were determined on xylose lysine deoxycholate (XLD) and tryptic soy agar supplemented with 0.6% yeast extract (TSAYE), respectively. Seeds using this procedure were drained and dried at 25, 51 and 60°C. The water activity (aw) and microbial population of seeds were monitored during drying. Seeds soaked and dried at 25oC were utilized for subsequently storage studies. Portions of dried chia seeds were ground into powder, then pumpkin, sunflower and chia seeds and chia seeds powder were stored at either 37 or 45oC at three different RH levels. The aw and microbial population of seed samples were monitored during storage. During the seed soaking, Salmonella and APC increased approximately 3 log CFU/g. Some decreases were observed at the end of the growth period, as may be expected after stationary phase. Drying at 60oC to a aw of 0.1 resulted in over 6-log CFU/g loss in bacterial population in partially sprouted pumpkin and sunflower seeds, and 5-log CFU/g loss in partially sprouted chia seeds. However, seeds dried at 25oC bring the aw to 0.5 had only approximately a 1-log CFU/g loss in bacterial population. The highest temperature (45oC) and RH (76%) storage condition had the greatest inactivation effect. At 45oC and 76%RH, Salmonella in pumpkin and sunflower seeds were below detection limit after 4 d and 7 d respectively. Salmonella populations in chia seeds and chia seeds powder were still detectable after 7 d regardless of the treatment condition. Salmonella is capable of survival during the manufacture of partially sprouted seeds including both drying and storage steps. Mild thermal treatment coupled with high RH storage condition holds promise for Salmonella inactivation in pumpkin and sunflower seeds but is less effective for chia seeds and chia seeds powder. Dried, partially sprouted seeds that are used as ingredients or consumed directly without further processing are a high-risk food product. Such products must be treated to eliminate foodborne pathogens that may cause foodborne illness.
M.S. in Food Science, May 2017
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SALMONELLA SURVIVAL DURING INOCULATION AND STORAGE OF DRIED SPICES CONTAINING ANTIMICROBIAL COMPONENTS
Title
SALMONELLA SURVIVAL DURING INOCULATION AND STORAGE OF DRIED SPICES CONTAINING ANTIMICROBIAL COMPONENTS
Creator
Hu, Chuxuan
Date
2016, 2016-05
Description
To provide an adequate initial microbial level for spices containing antimicrobial compounds for use in stability testing of foodborne...
Show moreTo provide an adequate initial microbial level for spices containing antimicrobial compounds for use in stability testing of foodborne pathogens, a dry-transfer inoculation was designed and compared with a traditional aqueous inoculation protocol. A five serovar cocktail of Salmonella spp. was prepared at different concentrations (~11 log CFU/mL, ~8 log CFU/mL and ~4 log CFU/mL). It was inoculated onto 1g silica beads and four types of dried ground spices (clove, oregano, ginger, and black pepper). Both spices and beads were dried for 24h at ambient conditions (~22°C). Silica beads were then used as a carrier to inoculate the same four types of spices. The results suggest that dry-transfer of Salmonella via inoculated silica beads provided a greater starting inoculum than aqueous transfer alone and are a viable alternative to aqueous inoculation when spices contain antimicrobial components. This dry-transfer inoculated ground clove with 8.4 log CFU/g and 3.9 log CFU/g initial populations was then used to determine the survival of Salmonella on spices containing antimicrobial components. Salmonella spp. survival on silica beads with 9.2 log CFU/g and 5.9 log CFU/g initial populations and subsequent transfer to ground clove was also examined. Samples were enumerated at selected time point up to 210 d. Results of this study indicate that Salmonella transfer and recovery from beads and in ground clove was not affect by storage and that Salmonella may persist for an extended period of time. The calculation of recovery was, however, affected by detection limits in clove. To improve detection limits an oil extraction was used and compared to the BAM method (detection limit 3.7 log CFU/g). After oil extraction the clove samples were soaked for 1 h (or 24 h for low inoculum concentration) in TSB at 37ᵒC and subsequently enumerated on tryptic soy agar with 0.6% yeast extract and xylose lysine deoxycholate agars. The result demonstrates that the oil extraction detection method holds promise as an alternative method to detect Salmonella in dry spices containing antimicrobial components and can provide a lower detection limit (1.7 log CFU/g).
M.S. in Food Process Engineering, May 2016
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MOLECULAR MECHANISMS UNDERLYING SALMONELLA SURVIVAL ON SURFACE OF SELECTED NUTS AND FRUITS
Title
MOLECULAR MECHANISMS UNDERLYING SALMONELLA SURVIVAL ON SURFACE OF SELECTED NUTS AND FRUITS
Creator
Li, Ye
Date
2017, 2017-07
Description
Salmonella enterica is the leading cause of bacterial foodborne illness in the United States. In recent years, S. enterica has been frequently...
Show moreSalmonella enterica is the leading cause of bacterial foodborne illness in the United States. In recent years, S. enterica has been frequently linked to foodborne outbreaks associated with nuts and fruits; however, the underlying mechanisms of such association have not been fully understood. In the first part of this study, we evaluated the impact of various environmental factors and food surface attributes on the attachment and survival of five S. enterica strains representing serotypes Typhimurium, Enteritidis, Montevideo, Mbandanka, and Braenderup on three different raw nuts (i.e. black peppers, almonds and hazelnuts) and two different S. enterica strains including serotypes Typhimurium and Enteritidis on two fresh fruits (i.e. grape tomatoes and cantaloupes) under storage conditions relevant to industrial practice. We observed significant inter-strain variations in S. enterica survival on nut and fruit surface. A direct correlation was found between the nut and fruit surface roughness and S. enterica attachment and survival. Lower relative humidity (20%) and higher storage temperature (25oC) resulted in significant S. enterica reduction on nut shells. Lower storage temperature at 4oC significantly reduced S. enterica population on grape tomatoes. In the second part of this study, we used a newly-developed transposon mutagenesis library in S. enterica serotype Enteritidis genome and highthroughput sequencing analysis to identify genes with potential roles in S. enterica attachment to and survival on almonds and grape tomatoes. A total of 336 and 210 S. enterica genes displayed significant selection on almonds and grape tomatoes over a 7-d storage period at 25oC (p<0.05), respectively. Our results suggest that various food attributes, environmental factors as well as bacterial determinants collectively contribute to the survival and persistence of S. enterica on nuts and fruits, providing new data for future development of knowledge-based intervention strategies.
Ph.D. in Biology, July 2017
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