Preservation methods for pure cultures

Preservation is a process to maintain pure culture for extended periods in a viable condition without any genetic changes. Once a pure culture has been obtained from the mixture, the microbes may be maintained in the laboratory over long periods. 

The most important factor of preservation is to stop microbial growth or at least suppress the growth rate.

Objectives of Preservation:

• To maintain isolated pure cultures for longer periods in a viable conditions. 

• To avoid contamination. 

• To restrict genetic changes.

Several techniques are used to preserve cultures:

Subculturing: In this method, nutrients of culture medium is regularly changed. So, culture media should be pure. Nutrient agar medium is commonly used where most of the heterotrophs remain viable for several months.

Paraffin method: In this method, paraffin oil added in the culture media, which inhibit the growth of bacteria, because it forms a thin layer over the surface of culture media. The layer of paraffin ensures anaerobic conditions and prevents dehydration of the medium. This condition helps microorganisms or pure culture to remain in a dormant state. 

The culture is preserved for several years.

This method is also known as Agar Slant Culture Covered with Oil.

Refrigeration: In this method, pure culture is placed under refrigerator in which temperature is about 0 - 4 degree C for some days. So, microbes don't Birth and other microbes were killed. 

This method is applied for short duration (2-3 weeks for bacteria and 3-4 months for fungi) because the metabolic activities of the microorganisms are greatly slowed down but not stopped.

Cryopreservation: Cryopreservation helps in survival of pure cultures for long storage times. In this method, report culture media at very low temperature [at about 150 - 196 degree C], then all the microbes were killed by dehydration.

Lyophilization: In this method, culture media put in very low temperature, then the pressure is reduced. So, microbial cells are dehydrated and their metabolic activities are stopped. After it sealed and stored in the dark at 4 degree C in refrigerator. The microbes go into dormant state and retain viability for years. 

_ This method is also called freeze dried pure cultures.

Saline suspension: Sodium chloride in high concentration is frequently an inhibitor of bacterial growth. Bacteria are suspended in 1% salt solution. The tubes are stored at room temperature. Whenever needed the transfer is made on agar slant.