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Freeze-thaw effect on selected fecal indicator bacteria : Escherichia coli and Enterococcus faecalis / by Nicole A. Hawdon.

dc.contributor.advisorGao, Wa
dc.contributor.advisorLeung, Kam Tin
dc.contributor.authorHawdon, Nicole Anita
dc.description.abstractThe survival of two selected fecal indicator organisms, two strains of Escherichia coli, a Gram-negative bacterium, and two strains of Enterococcus faecalis, a Gram-positive bacterium, after freezing and thawing successively for five cycles was determined using a drop plating method. It was found that all bacterial strains, when an initial concentration of 1.0 x E +08 was used, showed significant decreases in their ability to be cultured (p < 0.05) when frozen and thawed at -15C, compared to three other freezing temperatures, -7, -30 and -80C, when all four were frozen for 24 hours. In addition, the number of culturable cells at -7, -30, or -80C were not significantly different from each other (p > 0.05). The differences in cell inactivation between the two strains of each species of bacteria tested at all temperatures was not significantly different after five freeze-thaw cycles; while the difference between species was shown to be significant, depending on the temperature and condition tested (p < 0.05). When comparing small sample volume sizes (100l) to larger sample volume sizes (100ml) the observed differences were that Escherichia coli strains showed a decrease in cell culturability at both -7C and -15C when cycled in the larger volume; whereas, Enterococcus faecalis strains showed a decrease in cell culturability at -7C and an increase in cell culturability at -15C when cycled at the larger volume. Additional studies investigating culturability, cell wall integrity, and membrane damage of the bacterial strains were conducted using 100ml samples, cycled at -7, -15, and -30°C, and evaluated by three microbiological methods: drop plating, epi-fluorescent microscopy, and flow cytometry, respectively. In all instances, the plate counting method indicated that there was a decrease in cells that were culturable. Results from flow cytometry indicated a smaller decrease in cell culturability, followed, lastly, by results using the epi-fluorescent microscope. Thus, these studies would suggest that the most damage that occurs to frozen and thawed cells, when cycled five times at -7,-15 or -30°C, would be due to damages occurring at the cellular level, rather than damages occurring on the cell envelope, since less cells were able to uptake nutrients from the culture plates.
dc.subjectEnterococcus faecalis
dc.subjectEscherichia coli
dc.titleFreeze-thaw effect on selected fecal indicator bacteria : Escherichia coli and Enterococcus faecalis / by Nicole A. Hawdon.
dc.typeThesis University
dc.contributor.committeememberSchraft, Heidi
dc.contributor.committeememberProvidenti, Miguel

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