JSSS
Agar
Teacher support material
Nutrient agar in Petri dishes is an ideal growing medium for micro-organisms. Students may need agar plates for required coursework activity OB65.
Agar plates

Ready made sterile nutrient agar plates can be bought from scientific suppliers at approximately €1 per plate , with a guaranteed refrigerator shelf life of eight weeks

Buying nutrient agar powder and sterile Petri dishes to prepare the plates in the laboratory works out at approximately 30c per plate . Preparation of one batch of plates takes about 20 minutes; the refrigerator shelf life of the plates will vary greatly depending on the degree of sterilisation achieved.
Note: A local hospital laboratory may be willing to donate agar plates nearing their use-by date.
Incubators

Incubators from scientific suppliers cost from €500
 Electric seed propagators from garden centres cost from €40

A very cheap alternative is to use a biscuit tin lined with newspaper or polystyrene chips
The quantities here will make 500 cm
3
of agar, enough for approximately 25 plates.
1
Clear and clean bench.
Materials needed: nutrient agar powder, sterile small spatula, filter paper, distilled (or cooled, boiled) water, electronic balance, sterile Petri dishes,
Pyrex vessel – 500 cm 3 round bottomed flask used here, heat source – electric heating mantle used here, thermometer, protection for hands – here an oven glove is used, basin of cold water.
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JSSS Teacher support material
2 Weigh out 20g nutrient agar powder.
3
Place 500 cm
3
water into flask.
Add the agar powder to the water.
Stir until dissolved.
4. When fully dissolved, heat the agar solution until it boils.
5.
While the agar is heating, line up the Petri dishes along the edge of the bench.
6.
When the agar boils, remove from heat – use oven glove or suitable tongs.*
7.
Place agar solution in basin of cold water to cool to below
50°C before pouring.
Open each plate as little as possible, pour agar to cover the base of the plate (approx. 20 cm
3)
, close the lid.
8.
Allow the plates to solidify.
These plates are ready for immediate use.
Store upside down in a refrigerator until needed.
*If an autoclave or pressure cooker is available, the agar should be totally sterilised at this point
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