Proper storage of agar plates after inoculation is important for successful microbial cultivation. Correct post-inoculation handling helps ensure optimal conditions for target organism growth while minimizing the risk of unwanted microbial contamination. This careful approach maintains the integrity of experimental results and the viability of microbial cultures.
Preparing Plates for Storage
After inoculation, invert agar plates. Storing them upside down prevents condensation, which naturally forms on the lid during incubation, from dripping onto the agar surface. This condensation could otherwise spread microbial colonies, making isolation and counting difficult.
After inverting, seal plates to maintain moisture and prevent airborne contamination. Parafilm, a stretchy, self-sealing film, is commonly used to wrap around the edges of the petri dish, creating a tight seal. Alternatively, plastic wrap can provide a sufficient barrier against dehydration and external microbes.
Labeling each plate immediately after inoculation aids organization and accurate record-keeping. Essential information should include the date of inoculation, the type of microorganism cultured, and the experiment name or sample identification number. Clear and comprehensive labeling prevents mix-ups and ensures that specific cultures can be easily tracked throughout their storage and incubation period.
Choosing the Right Storage Environment
The appropriate temperature for storing inoculated agar plates depends on the desired outcome, whether it is incubation for growth or long-term preservation. Most common bacteria grow optimally between 25°C and 45°C, with many human pathogens thriving around 37°C. Fungi and many environmental bacteria often prefer slightly cooler conditions, typically incubating optimally between 25°C and 30°C.
Refrigeration slows microbial metabolism and extends culture viability. Storing plates at temperatures between 2°C and 8°C after initial growth significantly reduces microbial growth rates. This cold storage is suitable for maintaining cultures for several weeks to a few months without requiring frequent subculturing.
Adequate humidity is important, though sealing the plates largely addresses this concern. The seal created by parafilm or plastic wrap helps to contain the moisture within the petri dish, preventing the agar from drying out prematurely. Drying agar can inhibit microbial growth and reduce the overall lifespan of the culture.
Light negatively impacts agar media and certain microorganisms. Some media components, such as certain antibiotics, can degrade when exposed to light, potentially altering the selective properties of the agar. Furthermore, some bacteria and fungi are sensitive to light, and prolonged exposure can inhibit their growth or even kill them. Store plates in the dark or opaque containers for consistent growth and culture integrity.
Monitoring and Disposal
Visible microbial growth duration varies depending on the specific organism and incubation conditions. Many common bacteria show visible colonies within 18 to 24 hours of incubation. Fungi and some slower-growing bacteria may require several days, sometimes up to 2-3 weeks, to develop discernible growth.
Regularly inspecting plates allows for identification of target growth and any potential contamination. Desirable growth appears as distinct colonies with characteristic shapes, sizes, and colors. Contamination, often from airborne microbes, can manifest as spreading molds or different bacterial colonies than expected, indicating an issue with aseptic technique or environmental control.
For extended preservation, plates can be refrigerated after initial growth has been observed. While refrigeration slows metabolism, cultures stored this way will eventually decline in viability due to nutrient depletion and waste accumulation. For very long-term storage, cultures often need to be subcultured to fresh media periodically or preserved using methods like cryopreservation.
Proper disposal of inoculated agar plates prevents the release of potentially harmful microorganisms into the environment. In laboratory settings, autoclaving inoculated plates, which uses high-pressure steam to sterilize the biological material, is the standard method. For home or educational settings without an autoclave, soaking plates in a 10% bleach solution for several hours or overnight can effectively disinfect them before disposal in regular waste.