Petri Dishes

Petri dishes are useful for purposes such as performing cell counts, isolating strains of bacteria or fungi, testing for antibiotic properties or effects, and just about anything else that requires manipulation of microbes in a 2 dimensional enviroment. Pouring petri dishes usually involves pouring warm, autoclaved agar into presterilized dishes. Most petri dishes are about 4 inches in diameter by a little more than 1/2 an inch thick (100 X 15mm). The most commonly available petri dishes are the presterilized disposable polystyrene dishes. They are almost always the 4 inch wide type. These generally come prepackaged in units of 20 or 25. Nothing more needs to be done with them, except to pour the warm sterile agar into them. Glass petri dishes are also available, with or without metal covers. While these are more expensive to purchase in the beginning, they can be used over and over again. Glass petri dishes have the advantage of coming in several different sizes, from about 3 inches up to about 6 inches in diamter, with several different depths. Smaller petri dishes take less space to store, are cheaper, and use less agar. They are not quite as good for cell counting or some selective techniques however.

Now then, the trick is to get the sterile agar into the sterile petri dishes, without rendering the whole unsterile. Probably the single most important factor in accomplishing this is speed. The faster the plates are poured, the less chance there is for something to go wrong. Prepare everything the way you want it before you start. Have all your dishes, agar, and any other utensils you think you might need within arm's reach or yourself. Simplicity is the key ingredient to timely pouring of dishes. You might consider stacking all your empty plates in stacks of three.

This is only one way to pour them. Feel free to do what feels comfortable to you. Open up your agar container. Hold it in your favored hand, generally the right hand. Hold a stack in your offhand. Set the bottom dish in front of you, removing its cover and the top two petris. Pour just enough agar into the dish to completely cover the bottom. Quickly replace the cover of the dish, and in the same motion leave the bottom of the next dish sitting on the cover of the first poured dish. Now as you take your hand away you should have one complete dish and the cover of the working dish in your offhand. Pour the second dish right where it sits, on top of the first dish. Quickly cover it, again leaving a bottom plate sitting on top. Pour this third plate and cover it. Set this stack of three to the side and bring another stack in front of you and begin again. If you have large hands, you may feel comfortable with 4 or more dishes in a stack. If three feels cumbersome, try two at a time.

Here's a picture of a 4" plastic disposable petri dish nearly fully colonized with Agaricus brunnecens, a portabello strain, but with some sort of green mold contaminate growing at the edge. This is very likely to be some strain of Trichoderma, of the forest green mold. Trichoderma infestations cause huge losses to Pennsylvania mushrooms farms every year. For the home cultivator the best way to handle a contaminate like this is to culture the desirable species to a clean plate, rather than succumb to the temptation to try to cut the contaminate from the current plate.

Contaminated Petri Dish

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