2000 Volume 17 Issue 2 Pages 127-133
Viability of Escherichia coli O157:H7 during frozen storage at -20°C was measured by plating methods using Tryptic Soy Agar (TSA) and Cefixime-Tellurite-Sorbitol-MacConkey (CT-SMAC) agar. Viability of unwashed cells of exponentially-growing E. coli O157: H7 gradually decreased with an increase in storage period when the cells were suspended in water, 50 mM phosphate buffer (pH 7.0) and Tryptic Soy Broth. However, the viability decreased to only about 10% of that of the unfrozen cells after a 14-day frozen-storage in physiological salt solution. In contrast, the viability of the cells washed with water was the lowest when the cells were frozen-stored in physiological salt solution. Viability during frozen storage of both washed and unwashed cells of E. coli O157: H7 harvested at the stationary phase of growth was higher than that of the exponentially growing cells. When the cells in the exponential phase of growth were frozen-stored in phosphate buffers (pH6 .0) of various concentrations, the viability was the highest in 200mM buffer. Effects of pH on the viability of E. coli O157: H7 were examined after frozen-storage at-20°C for 3 days. Viability was the highest at pH6.5 in the cells of both exponential and stationary phases of growth after frozen storage. The viability of the washed cells was higher than that of the unwashed cells. The viability of E. coli O157: H7 was also measured by microscopy using LIVE/DEAD BacLightTM Bacterial Viability Kit. In the unwashed cells of the stationary phase of growth, the number of viable cells measured by the plating methods using TSA and CT-SMAC agar was 32% and 23% of that measured by microscopy using the kit, respectively, after storage in 50mM citric acid-phosphate buffer (pH6.5) at-20°C for 3 days. In contrast, in the washed cells, these numbers were 69% and 66% of that measured by microscopy, respectively.
