Once a pure culture is obtained then methods are to be devised for their maintenance and preservation so that all the characteristics can be conserved. Some of the simple methods of culture maintenance and their preservation are described below.
Use of refrigerator or cooling apparatus:
Live pure cultures can be successfully stored in their respective culture media in refrigerators or such cooling conditions at about 4°C. Generally, the metabolic activity of the organisms slows down and they become nearly inert at this temperature.
However, the metabolism does not completely cease and hence, the organisms cannot be maintained for an indefinite period of time. At regular intervals, say 2-4 weeks, the culture may be taken out from the refrigerator and inoculated to fresh media, a process known as sub- culturing.
Transfer to fresh media
Periodic transfer to fresh, sterile media tubes can maintain microbial cultures. The frequency of transfer, however, varies with the organism. A culture the bacterium, Escherichia coli, for example, needs transfer at monthly intervals. After growth for 24 hours at 37°C, the slants can be stored at low temperature for 20-30 days to keep the culture viable. It is necessary to use the appropriate growth medium and proper storage temperature
Overlying with mineral oil or liquid paraffin
Covering the fresh growth in agar slants with sterile mineral oil or liquid paraffin can preserve many bacteria and fungi. The oil must be above the tip of the slanted surface. The cell viability in this method is very high as compared to frequent transfer and storage at low temperature.
Freeze drying or lyophilization
Freeze drying (lyophilization) is a rapid dehydration of organisms while they are in frozen state. In this process, the cell suspension is placed in small vials, which are, frozen by immersing in a mixture of dry ice and acetone liquid nitrogen. The vials are evacuated and dried under vacuum, sealed and stored at low temperature. Under such conditions, microbes can be stored for very long durations without upsetting their characteristics.
Storage at sub-zero temperature
In this method, the cultures are frozen in the presence of a protective agent such as glycerol or dimethylsulphoxide in liquid nitrogen (-196° C). This method is successful in many organisms particularly those which cannot be preserved under lyophilization.
Storage in silica gel
Both bacteria and yeasts can be stored in silica gel at low temperature for 1 to 2 years. In this method, finely powdered, heat sterilized and cooled silica powder is mixed with a thick suspension of cells and stored at a low temperature. The quick desiccation at low temperature allows the cells to remain viable for a long period.