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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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