A Simple Method for Preserving Bacterial Cultures by Freezing and Drying

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Abstract
Bacteria maintain their original cultural, immunological and biochemical characteristics and their virulence for many years at room temp. when they are completely dried while in a frozen state, provided the cultures so dried are well sealed to keep out water vapor. Two methods are described for freezing and drying material. Both require the maintenance of a high vacuum until the cultures are dry and both employ P2O5 as a desiccant. In Method A the cooling system is a salt-ice mixture which is used both for freezing the cultures and for reducing the glycerol in the desiccator to a sub-zero temp., and some special provision must be made for keeping the cultures frozen until desiccation is complete. In Method B the cultures are frozen in the desiccator by immersion in glycerol to which solid CO2 is added; and suitably low temps. are maintained by placing the desiccator in a cork insulated box, containing a small amt. of CO2 ice. Either Picein or sealing wax makes satisfactory seals provided the tubes are never subjected to ice-box temps.