Temperature below the optimum for the growth decreases the rate of the metabolism but if it is sufficiently low, growth and the metabolism ceases. Low temperature is useful for the prevention of the cultures as micro organisms have a unique capacity of surviving the extreme cold. This is because below 0 degree celsius no life processes can occur. It is mainly due to the formation of ice crystals in the cells. These cut the membrane and coagulate the protein. Micro organisms are generally resistant to freezing, and may survive extreme cold. The initial exposure of the microbial population to the freezing temperature kills some of the organisms, but those which survive are capable of remaining viable for years. These organisms are considered as dormant, they do not perform any detectable metabolic activity. Low temperature can not be depended upon for sterilisation or disinfection. Generally low temperature is used for preservation of food, frozen food are therefore not sterile. The organisms are present and when conditions change and temperature rises, they are ready to grow.
Agar slant cultures of bacteria, yeasts and molds are stored for longer periods of time at refrigeration temperatures of 4 to 7 degree celsius. Many bacteria and viruses can be maintained in the deep freeze unit at temperature from -20 to -70 degree celsius . liquid nitrogen, at a temperature of -196 degree celsius is used to preserve cultures of bacteria, viruses and mammalian tissue cells to be used in research. Thus from practical stand point, high temperature is microbicidal and lo temperature is microbiostatic.