Preparation and Disposal of Nutrient Agar Plates
Introduction
This method may be used to prepare nutrient agar plates for short
term usage where meticulous sterility is not required. If plates are
to be kept for any length of time before use, then the agar solution
should be sterilised at 120°C in a pressure cooker.
The procedure for safe disposal of used agar plates is also
described
Procedure for Preparation
1. Weigh out 12.6g nutrient agar.
2. With gentle stirring on a hotplate-stirrer, slowly add the agar
to 450ml of cold demineralised water in an 800ml beaker.
3. When addition of the agar is complete, heat the mixture with
vigorous stirring and boil for 15 minutes.
4. Remove the beaker from the stirrer hotplate, retrieve the
stirrer “flea” using a magnet, and cover the beaker (foil).
5. Allow to cool to 50°C.
6. Pour the agar into clean Petri dishes.
7. Store the plates in the refrigerator (short term only).
Hint: Pour the agar to cover about ⅔ of the plate and then
gently tip the Petri dish to give a complete covering. In this way
there should be sufficient agar for a class set of 30 plates.
Procedure for Disposal (using a Tower brand domestic pressure cooker)
1. Set up the equipment as shown in figure 1.
Note: the “biohazard” bag is trimmed down so that it does
not interfere with the pressure relief mechanism in the lid of
the pressure cooker. The water is outside the bag.
2. Fit the lid of the pressure cooker.
3. Heat strongly on a gas ring until the correct pressure is
achieved – the indicator valve will show red. This may occur
when most of the water has boiled off and may take twenty
minutes or more from moment that steam starts escaping
from the relief valve. The gas can then be turned down to
the minimum required to keep the indicator showing red.
4. Continue heating to maintain the pressure for a further 15
minutes.
5. When cool, the plates may be disposed of as general waste.
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Figure 1
indicator valve
relief valve
lid
“biohazard” bag
pressure cooker
used plates
450ml water
small trivet
P.M. 3.12.2007
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