Survival curves for microbial species stored by freeze-drying

The survival of a variety of species of microorganism following storage for up to 20 years has been analyzed. The organisms were freeze-dried, sealed in ampoules under vacuum (<1 Pa) and stored in the dark at 5 degrees C. The yeast that was tested, Saccharomyces cerevisiae, showed only 8% survival when recovered shortly after freeze-drying, but subsequent loss during storage was the least among all the tested microorganisms. The decrease in the logarithm of survival per year (log survival) was -0.010, which corresponds to a survival rate of 97.7% per year. The Gram-negative bacteria tested, Escherichia coli, Pseudomonas putida, and Enterobacter cloacae, showed 42.6, 33.5, and 50.8% survival shortly after freeze-drying, which was higher than the corresponding survival of S. cerevisiae, but the subsequent loss during storage was greater than S. cerevisiae, the log survival figures being -0.041, -0.058, and -0.073 per year. These values correspond to survival rates of 91.0, 87.5, and 84.5% each year. The Gram-positive bacteria tested, Lactobacillus acidophilus and Enteroccoccus faecium, showed 62.5 and 85.2% survival shortly after freeze-drying, which was even higher than that of the Gram-negative species, and these organisms also showed better survival during storage than Gram-negative bacteria; their log survival rates were -0.018 and -0.016 per year, which corresponded to survival rates of almost 96% per year. Comparison of these results with other published data for different drying conditions suggests that survival during storage is strongly influenced by the degree of vacuum under which the ampoules were sealed. The excellent survival after freeze-drying of each species might be attributable to the high level of desiccation and to sealing under vacuum.