Preparing an Agar Plate

00:06 Speaker 1: You need to prepare nutrient agar plates for students to use to grow bacteria. Each group will need one plate. From the kit, you need the bottles of nutrient agar, a permanent marker, and the sleeves of sterile, unused Petri dishes. You need to supply a heat source, a pan of water, an oven mitt or other heat protection, a ruler, and a clock or a timer.

00:37 S1: The Petri dishes are sterile, and it is important to keep them sterile. You need a clean surface to work on. You can put down a previously unopened newspaper, or wash the table surface with soap and water or alcohol.

00:54 S1: Remove the Petri dishes from their package. Be careful to keep the dishes closed.

01:02 S1: You need to prepare one Petri dish for each group of four students, plus one control dish, and one demonstration dish for each class. For example, if you have eight groups of four students in each class, you need 10 prepared Petri dishes. You can label the Petri dishes ahead of time, or choose to have students label them right before they inoculate the agar in class. To label the dishes, turn them upside-down on the table so that the bottom half is on top. Use a ruler to draw two lines to divide the bottom into four sections.

01:47 S1: Number the sections one through four. If you write the numbers backwards, they will read properly when the Petri dish is right-side up.

02:12 S1: Now it's time to melt and pour the agar into the Petri dishes. Each 125 milliliter bottle of nutrient agar from the kit will be divided evenly among five Petri dishes. Loosen, but don't remove the caps on the bottles. Set them in a pan of simmering water for about 30 to 40 minutes. The agar needs to be completely melted before pouring. Make sure the level of water is above the level of agar in the bottle.

02:45 S1: Make sure that the agar is completely melted before pouring. To check, lift out a bottle, close the cap, and gently swirl the agar around looking for lumps. The bottles are hot, so use an oven mitt to take them out of the water. Do not shake the bottles. Shaking causes bubbles to form in the agar. If you see lumps, loosen the cap, and put the bottles back in the simmering water until you are sure that no lumps remain.

03:31 S1: Once you are sure that no lumps remain, take the bottles out of the simmering water and allow them to cool for a bit before you start to pour the agar. It might take five minutes or so. Spread five Petri dishes at a time on your clean surface. Avoid breezes.

03:53 S1: Lift or tilt the lid of each dish just enough to allow you to pour agar to fill the dish about half a centimeter deep.

04:06 S1: If necessary, tip and swirl the dishes gently to cover the entire bottom of the dish. Do not put the lid all the way on the Petri dish until the agar has cooled for a few minutes. This will reduce condensation on the lids. We'll pour three out of these five Petri issues here.

Preparing an Agar Plate

04:30 S1: Stack the finished Petri dishes in groups of 10. Typically, condensation will only form on the top dish of any stack. This stack is the class set if you have eight groups of four students. Allow the stack to remain at room temperature or put them in the refrigerator to solidify. As soon as they're solidified, turn them upside down so that any condensation that forms on the lids will not drip onto the agar surface.