Introduction
Microorganisms used for research, teaching, pharmaceutical production and environmental studies must be preserved in a viable and genetically stable form. These preserved organisms are known as stock cultures. The process of maintaining a pure culture for long periods without contamination or genetic change is called preservation.
Stock cultures are essential for studying microbial products, producing vaccines, maintaining genetically modified organisms and storing useful strains for future applications. Several laboratory methods are used to maintain microorganisms in a healthy and stable condition.
Methods for Maintenance of Lab Cultures
Common methods for preserving microorganisms for long-term use include:
- Periodic transfer to fresh media
- Storage at low temperature
- Storage in sterile soil
- Preservation under mineral oil
- Lyophilization (freeze drying)
1. Periodic Transfer to Fresh Media
This is the simplest and most widely used method in microbiology labs. The culture is grown on agar slants, incubated and then stored in a refrigerator. At regular intervals (every few weeks), a portion is transferred to a fresh medium.
Advantages
- Simple and requires no special equipment
Disadvantages
- High risk of contamination
- Frequent transfers may cause mutations
- Only suitable for short-term preservation
2. Storage at Low Temperature
Refrigeration
Live cultures can be stored at 4°C for a short duration (usually up to four weeks). After that, sub-culturing is required. This method suits routine laboratory work but not long-term storage.
Deep Freezing in Liquid Nitrogen
For long-term preservation, cultures are frozen in the presence of cryoprotective agents like glycerol or DMSO. These agents prevent ice crystal formation that could damage cells.
- Samples are frozen to –150°C and stored in liquid nitrogen at –196°C.
- Cells remain viable for 10–30 years.
- Little to no change in characteristics occurs.
3. Storage in Sterile Soil
This method is mainly used for sporulating microorganisms like Bacillus, Streptomyces, Aspergillus and Penicillium. Pure spores are mixed with sterile soil and stored in a refrigerator.
- Suitable for several months
- Low-cost method
4. Preservation by Overlaying Cultures with Mineral Oil
In this method, the surface of the culture on agar slant is covered with a layer of sterile mineral oil (liquid paraffin). The oil completely covers the growth, preventing dehydration and slowing metabolic activity.
Advantages
- Very simple and inexpensive
- Can preserve cultures for 15–20 years
- Useful for anaerobic microorganisms
Disadvantages
- Some changes in characteristics may still occur
- Contamination can occur if sealing is improper
5. Lyophilization (Freeze Drying)
Lyophilization is one of the best methods for long-term preservation of many microorganisms. It preserves cultures in a dry and stable form with minimal cell damage.
Procedure
- A dense cell suspension is prepared.
- The suspension is placed in small vials.
- It is frozen at –60°C to –78°C.
- The vials are connected to a vacuum system.
- The frozen water sublimates, leaving dehydrated cells.
- Vials are sealed under vacuum and stored.
Advantages
- Preserved cultures remain viable for 30+ years
- No need for sub-culturing
- Genetically stable and contamination-free
- Occupies minimal storage space
- Easy to transport to other labs
- Culture can be revived easily by rehydration
- Also used to preserve enzymes, toxins, sera and biological materials
Key Points
- Maintenance of lab cultures is essential for research, vaccine production and industrial microbiology.
- Methods vary based on organism type, storage duration and available facilities.
- Freeze drying and liquid nitrogen storage offer the longest preservation with minimal genetic change.
- Periodic transfer and mineral oil preservation are suitable for routine laboratory use.
Detailed Notes:
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