When you have developed good strains of microorganisms, it must be preserved properly, so that it could be used in future also. If the cultures have not properly been preserved, their characteristic features may decline after certain period.
Consequently production of product also gets decreased. Therefore, several methods have been developed for maintaining the organisms in viable conditions over a long period of time. These methods vary according to strains.
1. Agar Slant Culture :
Agar stabs or slants are prepared after autoclaving the medium. They are inoculated carefully with individual culture and incubated overnight. Then cultures are stored at 5 to -20°C. It must be sub-cultured at an interval of six months.
2. Agar Slant Culture Covered with Mineral Oil :
After incubation the cultures are incubated to facilitate the growth of organism. Then the cultures are covered with mineral oil to a depth of 1 cm above the slant. They are sub-cultured after the storage of about 1 year.
3. Storage in Saline Suspension :
Bacteria require salts for various metabolic activities. But at high concentration, salts act as bacteriostatic (temporary growth inhibitor). Hence, bacterial culture is preserved in 1% salt concentration in screw-capped tubes to prevent evaporation. The tubes are stored at room temperature.
4. Preservation by Drying in Vacuum :
The cultures are dried over CaCl2 in a vacuum, then stored in a refrigerator. At such conditions, the organisms survive for longer period than the air dried cultures.
5. Cryo-preservation :
The cultures are grown in culture tubes. Then 10-30% of a cryo-protective agent (e.g. glycerol j or dimethyl sulfoxide) is added into it so that it could protect the culture from cold sock. The culture is dispensed into ampoules. The ampoules are properly sealed and kept in liquid nitrogen at -176 to 196°C.
6. Lyophilisation or Freeze-drying :
In this method the microbial suspension is put in small ampoules and frozen through drying under vacuum. Vacuum drying results in sublimation of cell water. Then the ampoules are sealed off in vacuum. The lyophilised cultures are stored at 4°C. The cultures remain viable for about 10 years. By this method a large number of cultures are maintained without variation in their characteristics.