Oxidative stress and thermotolerance of Escherichia coli O157:H7 in salami
Blackman, Isabel Claire
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Fermented dry salami was associated with a food borne illness outbreak involving Escherichia coli O157:H7. For cooked salami, it is believed that by virtue of the heat processing that it is safe. No studies have been done on the thermotolerance of E. coli O157:H7 concerning oxidative stress exerted by an oxidative complex containing the compounds iron III chloride, adenosine diphosphate and ascorbic acid, which are similar to those found in cured meats. In this study the effect of the oxidative complex and ascorbic acid on the thermotolerance of two E. coli strains, EO139 and 380-94, was studied. In the first study, it was observed that the optimum concentration of the oxidative complex that enhanced thermotolerance at 60...C was in the ratio of 10µM:10µM:40µM iron III chloride, adenosine diphosphate and ascorbic acid, while 15:15:60 and 20:20:80 ratios were inhibitory to the strains. In the second study, when the ascorbic acid concentration was similar to that used in salami processing, it was observed that the ascorbic acid and ascorbic acid with the other two compounds did not enhance the thermotolerance of either strain, but rather the ascorbic acid injured the cells. In the same study, it was observed that the longer the exposure time of strain EO139 to the oxidative complex at 59...C the greater the inhibitory effect. Furthermore, it was observed that when grown in acidogenic media such as tryptic soy broth (TSB) with dextrose there was an inhibitory effect while when grown in TSB without dextrose there was no enhanced thermotolerance at 50...C and 59...C. In a validation study the erythorbate in cooked salami did not enhance thermotolerance of strains EO139 and 380-94. This indicates that the processing parameters used in cooked salami decreases the likelihood of survival of E. coli O157:H7 and that enhancement of thermotolerance due to oxidative stress is dependent on concentration of oxidative complex, temperature, duration of exposure and type previous exposure to acidic conditions.