Freeze-injury in bacteria.

The effects of freezing on bacterial cells have been studied essentially for three purposes. Early studies, conducted toward the end of the nineteenth century and in the early part of the twentieth century, were directed mostly toward qualitative estimation of the ability of a bacterial suspension to resume growth subsequent to freezing and thawing. Availability of liquid air, liquid hydrogen, and liquid nitrogen during this period made it possible for researchers to explore the effect of low temperature (as low as -190°C) on bacterial survival. Pictet and Yung' froze suspensions of several bacterial cultures at 120°C and stored them for 36 hr. They observed that after thawing, all the cultures survived and resumed growth. MacFadyen' reported that after freezing at 190°C for 20 hr, bacterial cultures not only survived but also retained their biochemical and other characteristics, viz., Staphylococcus aureus retained its pigment producing property and pathogens such as typhoid and diphtheria bacilli retained their pathogenicity. Gradually, it became apparent to workers in this field that not all bacterial species survived frozen storage and thawing equally well; thus, freezing could not be used for long-term preservation of bacterial cultures unless improved methods were developed. Studies were conducted to understand the mechanism by which freezing and thawing produced lethal effects on bacterial and other microbial cells. This probably initiated the quantitative estimation of lethality produced by freezing and thawing of bacterial cells. The effects of freezing and thawing on the viability loss of the bacterial cell under variable conditions were tested to explain the mechanism of death caused by freezing. It became clear that bacterial cells differed in their susceptibility to freezing and thawing according to strains, species, age, growth conditions, nature of the suspending medium, freezing and thawing conditions, and many others. Morphological, biochemical, and genetic studies were also conducted in an attempt to elucidate the possible changes brought about by freezing and thawing of bacterial cells, and to understand the underlying physical and chemical changes that occur during freezing of biological materials. This would hopefully be useful in devising means for successful freezing of higher forms of cells, including mammalian cells and organs. Bacterial