Agar is the jelly-like substance used to culture bacteria. It is made from a type of red algae, which provides an ideal growing surface for many different types of bacteria. Some types of agar contain added nutrients (such as sheep's blood) which help to promote more vigorous bacterial growth.  The easiest type of agar to use in this experiment is a nutrient agar which comes in powder form. You will need as much agar as you need, but don't use less than 1.2 grams (½ teaspoon) of agar powder for every 10 centimetres (3.9 in) Petri dish you wish to use. In a heatproof dish or bowl, stir 1.2 grams (½ teaspoon)  of the nutrient agar powder into 60 millilitres (0.25  c) of hot water. Multiply these quantities by however many Petri dishes you plan on using. Place the bowl or dish in the microwave, and let it begin to boil for 1 or more minutes, watching to make sure that the agar solution doesn't boil over. When the solution is ready, the agar powder should be completely dissolved and the liquid should be clear in color. Allow the agar solution to cool for several minutes before proceeding.You don't want to get burnt! Petri dishes are small flat-bottomed containers made of clear glass or plastic. They have two halves - a top and a bottom - which slot into one another. This protects the contents from any unwanted contaminated air, but also allows any gasses produced by the bacteria to escape.  Petri dishes must be completely sterilized before they are used for growing bacteria, otherwise, the results of the experiment could be affected. Newly purchased Petri dishes should come pre-sterilized and sealed in plastic packaging. Remove the Petri dish from its packaging and separate the two halves. Very carefully, pour the warm agar solution into the bottom half of the Petri dish - just enough to form a layer over the bottom of the dish. Work in the presence of a candle with a tall flame or a Bunsen burner to keep contamination low. Quickly replace the top half of the Petri dish to prevent any airborne bacteria from contaminating the experiment. Set the Petri dishes aside for 30 minutes to 2 hours, until the agar solution cools and hardens (when it’s ready it will resemble set Jell-O). If you don't plan on using the agar-filled Petri dishes immediately, they should be stored in the refrigerator until you are ready to proceed with the experiment.  Storing the Petri dishes in the refrigerator prevents the water inside the dishes from evaporating (bacteria need a moist environment to grow). It also allows the surface of the agar to harden slightly, which prevents any tearing or gouging when you transfer your bacteria samples. When storing Petri dishes in the refrigerator, the dishes should be placed upside down. This helps to prevent any condensation on the lid from dropping down and disrupting the growing surface. Agar-filled Petri dishes will keep in the refrigerator for as long as a couple of months. When you are ready to use them, remove them from the refrigerator and allow them to reach room temperature before introducing your samples.
جهز الأجار. جهز أطباق بتري. برد أطباق بتري لحين تستعد لاستخدامها.