JSSS
Teacher support material
Agar
Nutrient agar in Petri dishes is an ideal growing medium for micro-organisms. Students may
need agar plates for required coursework activity OB65.
Comparing costs (prices current December 2005)
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
Preparing nutrient agar
The quantities here will make 500 cm3 of agar, enough for approximately 25 plates.
Clear and clean bench.
Materials needed:
nutrient agar powder,
sterile small spatula,
filter paper,
distilled (or cooled, boiled) water,
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electronic balance,
sterile Petri dishes,
Pyrex vessel – 500 cm3 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
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Teacher support material
Weigh out 20g nutrient agar powder.
Place 500 cm3 water into flask.
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Add the agar powder to the water.
Stir until dissolved.
4.
5.
6.
When fully dissolved, heat the agar solution until it boils.
While the agar is heating, line up the Petri dishes along
the edge of the bench.
When the agar boils, remove from heat – use oven glove
or suitable tongs.*
Place agar solution in basin of cold water to cool to below
7.
50°C before pouring.
Open each plate as little as possible, pour agar to cover
the base of the plate (approx. 20 cm3), close the lid.
Allow the plates to solidify.
8.
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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