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Survival and Persistence of dried Salmonella enterica in Low Water Activity Conditions

  • Author / Creator
    Seeras, Arisha
  • Salmonella survives in low moisture products over an extended time. Salmonella also resists dry heat, which questions the assumption that Salmonella outbreaks from intermediate moisture foods result mostly from post-processing contamination. Some Enterobacteriaceae possess a genomic island called the locus of heat resistance (LHR), which confers heat resistance. This genomic island was also identified in Salmonella Senftenberg ATCC 43845, a strain with exceptional heat resistance. This study aimed to determine whether the locus of heat resistance improves survival of Salmonella during storage in low-moisture environments, and its resistance to dry heat. The LHR-positive S. enterica Senftenberg ATCC 43845 and the LHR-negative S. enterica Typhimurium ATCC 13311 were air dried and stored for up to 30 d at a relative humidity (RH) of 10%, 55%, or 75%. Saturated salt solutions were used to maintain a constant RH throughout storage. All experiments were conducted in triplicate and analyzed using one-way ANOVA. Cell counts of both strains of Salmonella were reduced by less than 1 log (CFU/g) after 30 d of storage at 10% RH. At 75% RH, the viability of the LHR-positive S. enterica ATCC 43845 was reduced by more than 8 log (CFU/g) after 3 d of storage, while viability of the LHR-negative S. enterica ATCC 13311 was reduced only by 2 log (CFU/g) after 3 d of storage. To further confirm the effect of the LHR, S. enterica ATCC 13311 was transformed with a plasmid pRK767 as a control or with pLHR, a plasmid obtained by cloning the LHR under control of the native promotor in pRK767. Cell counts of the LHR-complemented S. enterica ATCC 13311pLHR were reduced by 8 log (CFU/g) after 3 d of storage at 75% RH, which is comparable to results for the LHR-positive wild type strain, and cell counts of the LHR-negative S. enterica ATCC 13311pRK767 were reduced only by 3 log (CFU/g). Survival under low RH conditions of an isogenic strain without kefB (S. enterica ATCC 13311pLHRΔkefB) indicated that kefB might have a specific role during desiccation. Resistance to dry heat was determined by treatment at 110°C for 15 min. At a RH of 55% or lower, cell counts of all strains were reduced by less than 1 log (CFU/g). However, cell counts of the LHR-positive S. enterica ATCC 43845 and S. enterica ATCC 13311pLHR were reduced by 8 log (CFU/g) after heating to 110°C at 75% RH while the LHR-negative S. enterica ATCC 13311 and S. enterica ATCC 13311pRK767 were reduced only by 6 log (CFU/g). In conclusion, dried cells resist dry heat after equilibration in environments with a RH of less than 60% irrespective of the presence of the LHR. The LHR increases resistance to wet heat but reduces survival when cells are dried and reduces the heat resistance of dried cells.

  • Subjects / Keywords
  • Graduation date
    2017-06
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/R3TM72C98
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
    English
  • Institution
    University of Alberta
  • Degree level
    Master's
  • Department
    • Department of Agricultural, Food, and Nutritional Science
  • Specialization
    • Food Science and Technology
  • Supervisor / co-supervisor and their department(s)
    • McMullen, Lynn M./Ganzle Michael (Agriculture Food and Nutritional Science)
  • Examining committee members and their departments
    • Mohandas, Roopesh S. (Agriculture Food and Nutritional Science)