Petri Plate Drying
How to Dry Agar Plates Aseptically
(This procedure will take about one hour.)
- You have already poured your plates, and have possibly stored them right-side up (Fig 1 and not like in Fig 2) in the refrigerator. By now lots of condensation has accumulated in them that must be aseptically removed.
- Find a countertop that is free of drafts and insects (ants, flies, roaches, etc.) which like to walk around on the agar surfaces with their dirty feet.
- You need a nearly sterile surface. Here's a cheapie way to make one. Consider a fresh roll of paper towels. The paper inside is essentially sterile due to its hot, untouched, manufacturing process.
- Touching only the edges or the outside surface, unroll a length of paper towels, and lay these out on your countertop such that the inner surface is up. Voila: a sterile surface!
- Place a prop such as a pencil down on the paper towel near the end.
- Next open one plate at a time and set them one on the other as shown in Fig 3. Their openings MUST be pointing downwards so that dust will not settle in them. (If the covers have lots of moisture, fling off the excess by holding the cover tightly and giving it a quick flick of the wrist. Don't do this with the agar-filled bottoms unless you want to clean up a mess!)
- When you no longer see liquid on the inside of the covers and on the agar surfaces, put the plates together and stack them right side up (Fig 1). (This prevents dust from falling into the joint. Were the plates stacked upside down (Fig 2), any dust that falls into the joint would then slip inside and, when the plate is turned over to use, would then fall onto the agar surface. Mold will most certainly grow.
Only once inoculated* are plates stored upside down (Fig 2).)
*Inoculate has only one "n". If your teacher catches you spelling it 'innoculate,' you will be condemned to bringing snacks the next day for the whole class, and you will not be able to eat any - just watch. Talk about being cruel and unusual punishment! (Well, it's still better than being thrown to the lions which are kept in the basement of the school.)
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