Preparing a petri dish involves pouring a sterile agar solution into the dish and allowing it to solidify, creating a surface for microbial growth.
Here's a step-by-step guide on how to prepare a petri dish:
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Prepare the Agar: This typically involves dissolving a pre-made agar powder in distilled water according to the manufacturer's instructions (usually by heating). Common agars include Luria-Bertani (LB) agar for bacterial growth and Sabouraud dextrose agar for fungal growth. The concentration of agar is usually around 1.5-2%.
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Sterilize the Agar: This is crucial to prevent contamination. Autoclaving is the most common method. Autoclave the agar solution at 121°C (250°F) for 15-20 minutes. Alternatively, you can use a pressure cooker.
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Cool the Agar (Slightly): After autoclaving, let the agar cool to around 50-55°C (122-131°F). This temperature is low enough to handle the flask and minimize condensation, but high enough to keep the agar from solidifying prematurely. You can place the flask in a water bath to maintain this temperature.
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Pour the Agar into Petri Dishes: Work in a sterile environment, such as near a Bunsen burner or in a laminar flow hood, to minimize contamination. Quickly lift the lid of a sterile petri dish and pour enough agar to just cover the bottom, typically 10-15 ml for a standard 90-100mm dish.
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Swirl for Even Distribution: Gently rotate the dish to ensure the agar covers the entire bottom surface evenly.
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Let the Agar Solidify: Immediately cover the dish and allow the agar to solidify completely at room temperature. This usually takes about 30-60 minutes. Do not move the dishes during this time.
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Invert and Store: Once solidified, invert the petri dishes and store them in a cool, dark place (e.g., a refrigerator at 4°C) to prevent condensation from accumulating on the agar surface. Inverting minimizes condensation onto the agar surface and prevents the media from drying out too quickly. Prepared plates can typically be stored for up to a few weeks.
Important Considerations:
- Sterility is paramount: Always use sterile materials and techniques to avoid contamination.
- Agar concentration: Using the correct agar concentration is important. Too low and the agar may not solidify properly, too high and it may be too firm.
- Additives: Antibiotics or other supplements can be added to the cooled agar before pouring to create selective media. Ensure any additives are sterile filtered before adding to the agar.
- Condensation: Excessive condensation can promote unwanted bacterial growth. Make sure the agar is not too hot when pouring, and invert the dishes during storage.
