Mechanism of injury of Escherichia coli by freezing and thawing

Freezing and thawing of Escherichia coli B/r has two types of lethal effect, (i) immediate, i.e. that occurring during freezing at −20°C or −196°C and immediate thawing, and (ii) delayed, i.e. that occurring during frozen (−20°C) storage. Alkaline sucrose gradient (5–20%) centrifugation revealed 1 to 6 single strand breaks (SSB) in the [ 3H] DNA from frozen E. coli cells. The number of SSB showed a quantitative correlation with the extent of cell death, immediate as well as delayed. On this basis we conclude that cell death of E. coli by freezing, cold storage, and thawing is due to DNA degradation and loss of its vital integrity.