Petri dishes are shallow, transparent, lidded containers, made from glass or plastic. These dishes are fundamental tools in scientific and hobbyist settings, providing a controlled environment for culturing microorganisms and cells. They allow for observation and study of biological samples in a contained space, essential for a wide range of experiments.
Why Sterilization Matters
Sterilizing petri dishes ensures the culture medium and dish remain free from unwanted microbial life. Contamination from bacteria, fungi, spores, or viruses can invalidate experiments, leading to inaccurate results or obscuring desired cultures. Proper sterilization eliminates all microbial life, preventing cross-contamination and ensuring only target organisms grow.
Heat Sterilization Techniques
Autoclaving sterilizes glass petri dishes using high-pressure steam. This process involves subjecting dishes to 121°C (250°F) at 15 pounds per square inch (psi) for 15 to 20 minutes. Before autoclaving, clean and air-dry dishes, then wrap them in autoclave bags or aluminum foil to maintain sterility. If sterilizing dishes with liquid media, loosen lids to allow steam penetration and prevent pressure buildup.
Dry heat sterilization, using a conventional oven, is effective for heat-resistant glass petri dishes. This technique requires higher temperatures and longer exposure times than autoclaving due to less efficient dry air heat transfer. Heat dishes at 160°C (320°F) for two hours, or 170-180°C (338-356°F) for one to two hours. Dishes should be clean, dry, and wrapped in aluminum foil to protect them from dust. Always use heat-resistant gloves when handling hot glass from either method to prevent burns.
Maintaining Sterility
After heat sterilization, allow petri dishes to cool slowly within the sterilizer or oven. This gradual cooling prevents thermal shock and maintains sterility by preventing rapid air influx. For autoclaved dishes, ensure pressure and temperature have dropped before opening the chamber. Once cooled, store sterilized petri dishes in a clean, dry, dust-free environment, ideally in sealed bags or containers.
Refrigeration at 4°C (39°F) extends their shelf life and maintains sterility. When handling dishes, minimize air exposure by opening them only when necessary and closing them promptly. Working in a clean space, such as a laminar flow hood, and using sterile gloves prevents re-contamination. Storing dishes upside down prevents condensation from dripping onto the agar surface, which could promote unwanted growth.