Survival Differences of Escherichia coli O157:H7 Strains in Apples of Three Varieties Stored at Various Temperatures

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Abstract
Differences in survival and growth among five different Escherichia coli O157:H7 strains in three apple varieties were determined at various temperatures. Jonathan, Golden Delicious, and Red Delicious apples were wounded and inoculated with E. coli O157:H7 strains C7929 (apple cider isolate), 301C (chicken isolate), 204P (pork isolate), 933 (beef isolate), and 43890 (human isolate) at an initial level of 6 to 7 log CFU/g. The inoculated apples were stored at a constant temperature of 37, 25, 8, or 4°C or at 37°C for 24 h and then at 4°C, and bacterial counts were determined every week for 28 days. By day 28, for Jonathan apples at 25°C, the apple isolate counts were significantly higher than the chicken and human isolate counts. At 4°C for 28 days, the human isolate inoculated into Jonathan, Golden Delicious, and Red Delicious apples was present in significantly smaller numbers than the other strains. The apple isolate survived significantly better at 4°C, yielding the highest number of viable cells. By days 21 and 28, for apples stored at 37°C for the first 24 h and then at 4°C, the counts of viable E. coli O157: H7 apple and human isolates were 6.8 and 5.8 log CFU/g at the site of the wound, whereas for apples kept at 4°C for the duration of storage, the respective counts were 5.6 and 1.5 log CFU/g. Our study shows that E. coli O157:H7 strains responded differentially to their ability to survive in these three apple varieties at 25 or 4°C and produced higher viable counts when apples were temperature abused at 37°C for 24 h and then stored at 4°C for 27 days.