Petri dishes provide a controlled environment for cultivating microorganisms like bacteria, yeasts, and molds, making them a valuable tool for home science observations. Creating your own agar dishes requires careful hygiene throughout the process.
Gathering Your Supplies
To prepare agar petri dishes at home, you will need specific items. These include petri dishes (reusable glass or disposable plastic), agar-agar powder (a solidifying agent derived from red algae), and sterile water (distilled or boiled). A heat source, such as a microwave or stovetop, and measuring tools are also necessary. For proper sterilization, a pressure cooker is highly recommended, though alternatives exist.
Creating the Agar Medium
The process begins by accurately measuring the agar powder and water according to your chosen recipe. A common ratio is about 15 grams of agar powder per liter of water, or roughly one level teaspoon of agar for every 100 milliliters of water. It is important to mix the agar powder into room temperature water before heating, as adding it to warm or hot water can cause clumping. Once combined, heat the mixture, stirring constantly, until the agar fully dissolves and the solution becomes clear.
Sterilizing the agar medium prevents unwanted microbial growth. The most effective method uses a pressure cooker, reaching 121°C (250°F) at 15 psi for 15 to 20 minutes. Ensure the container holding the agar mixture is not fully sealed to allow proper pressurization and prevent hazards. Without a pressure cooker, boiling the agar mixture for at least 5 minutes, or two hours in a boiling water bath, can sterilize it, though less effectively.
Filling and Setting the Dishes
After sterilizing, allow the agar medium to cool slightly before pouring. The ideal pouring temperature is between 50°C and 60°C, or when the container is comfortable to hold; if too hot, condensation may form on the petri dish lids. Pouring should occur in a clean environment, such as near an open flame, to create an upward air current that minimizes airborne contaminants. Carefully lift the petri dish lid minimally and pour the warm agar, just enough to cover the bottom of the dish (about 1/8 to 1/4 inch deep).
Immediately replace the lid after pouring to maintain sterility and prevent contamination. Gently swirl the dish to ensure the agar evenly covers the base and to disperse any air bubbles. The agar will solidify within 30 minutes to one hour at room temperature, forming a firm, jelly-like surface. Once fully set, the dishes are ready for use or storage.
Safe Handling and Storage
Proper handling and storage of prepared petri dishes are important for maintaining their sterility and the success of your experiments. Store unused, prepared petri dishes upside down in a refrigerator. This inverted position prevents condensation from dripping onto the agar surface, which could create a moist environment conducive to contamination. Keeping the dishes sealed, perhaps in their original packaging or a clean plastic bag, prevents the agar from drying out.
After an experiment, especially once microbial growth is visible, handling and disposal require careful consideration. Never open petri dishes that contain visible microbial growth, as the microorganisms could be potentially harmful. For disposal, used petri dishes and their contents must be sterilized again to prevent the release of microorganisms into the environment. This can be achieved by autoclaving the dishes at 121°C and 15 psi for 15 to 30 minutes, or by soaking them in a 10% bleach solution for 20 minutes before discarding in regular trash, following local regulations.