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Effect of bacterial stress response on pathogen enumeration and its implications for food safety Open Access


Other title
bacterial stress response
food safety
pathogen enumeration
Type of item
Degree grantor
University of Alberta
Author or creator
Wang, Huaiyu
Supervisor and department
Bressler, David (Agricultural, Food and Nutritional Science)
Examining committee member and department
McMullen, Lynn (Agricultural, Food and Nutritional Science)
Raivio, Tracy (Biological Sciences)
Gänzle, Michael (Agricultural, Food and Nutritional Science)
Department of Agricultural, Food, and Nutritional Science

Date accepted
Graduation date
Master of Science
Degree level
To determine the impact of stress response on enumeration, cell association status and the viability of Escherichia coli DH5α, Staphylococcus aureus ATCC 13565 and Listeria monocytogenes CDC 7762 were evaluated using fluorescence microscopy and were compared with the outcomes of traditional plate count and optical density measurements. Fluorescence microscopy revealed that organic acid stress (acetic and lactic, pH 2.7-3.3) induced cell clumping with little loss of viability in Escherichia coli DH5α. Significantly lower values for cell enumeration were found for plate counts and OD600 measurement, likely due to cell clumping in response to organic acid stress. Gram-negative bacteria Escherichia coli DH5α showed higher levels of clumping and subsequent resistance against organic acid stress. Increased cell surface hydrophobicity was found in cells that exhibited more evident clumping. However, inorganic acid stress (hydrochloric and sulfuric, pH 3.0-3.3) induced only very low level of clumping in stationary-phase Escherichia coli DH5α and almost no clumping in other cultures. Osmotic stress, heat and cold shock were not found to induce cell clumping. It has been determined that traditional enumeration methods have significantly underestimated the number of viable bacterial cells when organic acid stress is involved. Plate counts and OD600 measurement therefore need to be reassessed as tools for accurate evaluation of pathogens in food industry.
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