Microbial Culture : Definition & Methods of Preserving Culture | Microbiology

A microbiological culture, or microbial culture, is a method of multiplying microbial organisms by letting them reproduce in predetermined culture media under controlled laboratory conditions. 

Methods of Preserving Microbial Culture:

The following points highlight the top five methods of preserving microbial culture. The methods are: 1. Agar Slant Cultures 2. Agar Slant Culture Covered with Oil (Parafin Method) 3. Saline Suspension 4. Preservation at Very Low Temperature 5. Preservation by Drying in Vacuum 6. Preservation by Freeze Drying (Byophilization).

1.Agar Slant Cultures:

All microbiology laboratories preserve micro-organisms on agar slant. The slants are incubated for 24hr or more and are then stored in a refrigerator. These cultures are periodically transferred to fresh media. Time intervals at which the transfers are made which varies with the origin and condition of growth.

2. Agar Slant Culture Covered with Oil (Parafin Method):

The agar slants are inoculated and incubated until good growth appears. They are then covered with sterile mineral oil to a depth of 1 cm above the tip of slant surface. Transfers are made by removing a loop full of the growth, touching the loop to the glass surface to drain off excess oil, inoculating a fresh medium and then preserving the initial stock culture.

This is a simple and most economical method of preserving bacteria and fungi where they remain viable for several years at room temperature. The layer of paraffin prevents dehydration of the medium and by ensuring an aerobic condition, the microorganism remain in dormant state.

3. Saline Suspension:

Sodium chloride in high concentration is frequently an inhibitor of bacterial growth. Bacteria are suspended in 1% salt solution (sublethal concentration in screw cap tubes to prevent evaporation). The tubes are stored at room temperature. Whenever needed the transfer is made on agar slant.

4. Preservation at Very Low Temperature:

The organisms are suspended in nutrient broth containing 15% glycerol. The suspension is frozen and stored at -15°C to -30°C. The availability of liquid nitrogen (temp -196°C) provides another main preserving stock culture. In this procedure culture are frozen with a protective agent (glycerol or dimethane sulphoxide) in sealed ampoules. The frozen culture are kept in liquid nitrogen refrigerator.

5. Preservation by Drying in Vacuum:

The organisms are dried over calcium chloride in vacuum and are stored in the refrigerator.

6. Preservation by Freeze Drying (Byophilization):

In this process the microbial suspension is placed in small vials. A thin film is frozen over the inside surface of the vial by rotating it in mixture of dry ice (solid carbon dioxide) and alcohol, or acetone at a temperature of −78oC .The vials are immediately connected to a high vacuum line. This dries the organism while still frozen. Finally, the ampules are sealed off in a vacuum with small flame.

These culture can be stored for several years at 40°C. This method is also employed for preservation of toxins, sera, enzymes and other biological material. To revive microbial cultures it is merely necessary to break open the vial aseptically, add a suitable stale medium, and after incubation make further transfers.

The process permits the maintenance of longer number of culture without variation in characteristics of the culture and greatly reduces the danger of contamination.

Depending on the chemical constituents from which they are made, their physical nature and their functions, different parameters of media are described here.