The metabolic activities of micro-organisms may be reduced considerably by storage at the very low temperatures (—150° to —196°) which may be achieved using a liquid nitrogen refrigerator. Snell (1991) claimed that this aproach is the most universally applicable of all preservation methods. Fungi, bacteriophage, viruses, algae, yeasts, animal and plant cells and tissue cultures have all been successfully preserved. The technique involves growing a culture to the maximum stationary phase, resuspending the cells in a cryoprotective agent (such as 10% glycerol) and freezing the suspension in sealed ampoules before storage under liquid nitrogen. Some loss of viability is suffered during the freezing and thawing stages but there is virtually no loss during the storage period. Thus, viability may be predictable even after a period of many years. Snell (1991) suggested that liquid nitrogen is the method of choice for the preservation of valuable stock cultures and may be the only suitable method for the long term preservation of cells that do not survive freeze-drying. Although the equipment is expensive the process is economical on labour. However, the method has the major disadvantage that liquid nitrogen evaporates and must be replenished regularly. If this is not done, or the apparatus fails, then the consequences are the loss of the collection.
Storage in a dehydrated form
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