The Correlation Between Tolerance To Low Water Activity And High Pressure Processing In Toxigenic Escherichia Coli

Date and Time


Food Science Building - room 239 (next to lecture room 241)



Final Examination for the Degree of PhD Food Science - FAHAD BAZAID

Examining Committee

Dr. Art Hill, Chair 
Dr. Keith Warriner, Advisor 
Dr. Tatiana Koutchma, Advisory Committee Member  
Dr. Jeff Farber, Department Member 
Dr. Siyun Wang, External Examiner (Skype) 
ABSTRACT:  High hydrostatic pressure processing (HHP) is extensively applied for the non-thermal pasteurization of foods. One notable feature of HHP mediated inactivation of microbes is the large variation in lethality between strains and the influence of the food matrix. In the following it was hypothesized that the bartolerance of Shiga Toxin producing Escherichia coli could be correlated to persistence under low water activity environments. The study evaluated the bartolerance of members of the Top 6 STEC serotypes and found variation with O26 and O45 exhibiting higher tolerance to pressure compared to the other bacterial types tested. From trials performed, it was found that the thermal tolerance of the strains did not correlate to bartolerance. However, a negative correlation was found between resistance to high pressure and sodium chloride concentration under which the test E. coli strains could grow. For example, E. coli O26 exhibited the highest tolerance to pressure but could only grow up to a NaCl concentration of 2.4%. In contrast, E. coli O121 was sensitive to pressure, but could grow in the presence of 6% NaCl. The results suggested that the induction of the stress-response was the likely reason for high bartolerance in salt sensitive strains vs resistant. It was also found that supplementing the suspension medium with amino acids, sugar alcohols and starch could increase the bartolerance of E. coli. Although it is possible that the solutes acted directly as osmolytes it is also proposed that metabolic products and other indirect protective mechanisms were also operating.  The study has illustrated for the first time that the bartolerance of STEC can be enhanced by inclusion of solutes and the ability to grow under high salt concentrations. Evidence would suggest the substrates act as an energy source although osmoprotection is also a possible mechanism. 

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