Agar plates provide a solid nutrient medium for growing mycelium, the vegetative part of a fungus. Proper storage maintains the mycelial culture for future use and prevents contamination.
Establishing the Right Environment
Storing mycelial cultures at reduced temperatures, typically between 2 and 8 degrees Celsius (35 to 46 degrees Fahrenheit), slows their metabolic processes. Lower temperatures decrease the rate at which the mycelium consumes nutrients from the agar and produces waste products. This prolonged nutrient availability allows the culture to remain viable for longer periods without needing a transfer.
Light conditions also affect stored mycelium. Exposure to light, especially direct sunlight or strong artificial light, can sometimes induce sporulation in certain fungal species, which might not be desired for mycelial storage. Additionally, prolonged light exposure can degrade some light-sensitive components within the agar medium, potentially reducing its nutritional value over time. Therefore, storing plates in a dark environment or in opaque containers helps maintain the integrity of both the mycelium and the growth medium.
Securing Plates Against Contamination
Protecting agar plates from external contamination is important for storage. Before storage, ensure plates are contaminant-free using aseptic techniques during their preparation and inoculation. Working in a clean environment, such as a laminar flow hood, helps minimize the introduction of airborne spores or bacteria that could compete with or destroy the desired mycelial culture.
Once the mycelium has adequately grown, sealing the plates helps maintain their sterility and prevent desiccation. Parafilm, a stretchable, self-sealing film, is frequently used to wrap around the edges of petri dishes. This creates a barrier that prevents airborne microorganisms from entering the plate and also reduces the rate at which moisture evaporates from the agar. Alternatively, cling wrap or specialized sealing tapes can also be used to achieve a similar protective seal around the plate, ensuring the internal environment remains stable and uncontaminated during storage.
Extending Mycelium Viability
The duration for which mycelium remains viable on agar plates depends on several factors, including the fungal species, nutrient composition of the agar, and storage conditions. For short-term storage, typically a few weeks to a few months, refrigeration is generally sufficient to slow growth and preserve the culture. During this period, the mycelium slowly consumes the available nutrients, and its metabolic activity is significantly reduced.
For longer-term preservation, exceeding several months, the mycelium may eventually exhaust the nutrients in the agar or accumulate waste products to inhibitory levels. In such cases, periodic transfers to fresh agar plates become necessary to maintain the culture’s vigor. The frequency of these transfers varies by species but can range from every 3-6 months to once a year. The inherent genetic characteristics of a specific fungal strain, such as its growth rate and metabolic efficiency, also influence how long it can persist on a single plate before needing a refresh.