Survival rate of microbes after freeze-drying and long-term storage.

The survival rates of 10 species of microorganisms were investigated after freeze-drying and preserving in a vacuum at 5 degrees C. The survival rates varied with species. The survival rates immediately after freeze-drying were different among yeast, gram-positive bacteria, and gram-negative bacteria, and the change in the 10-year survival rate was species-specific. The survival rate of yeast, Saccharomyces cerevisiae, was about 10% immediately after drying, and the rate did not decrease significantly during the 10-year storage period. Survival rates after the drying of gram-positive bacteria, i.e., Brevibacterium flavum, B. lactofermentum, Corynebacterium acetoacidophilum, C. gultamicum, and Streptococcus mutans, were around 80%. The survival rate of Brevibacterium and Corynebacterium did not decrease greatly during the storage period, whereas the rate of S. mutans decreased to about 20% after 10 years. Survival rates after the drying of gram-negative bacteria, i.e., Escherichia coli, Pseudomonas putida, Serratia marcescens, and Alcaligenes faecalis, were around 50%. The survival rate decreased for the first 5 years and then stabilized to around 10% thereafter.