More about microbes
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Preparation of media and cultures
Culture media
The method for the preparation of basic microbiology media is given below. In situations where preparation is uneconomic in time, prepared, sterilized media (liquid and solid) are available from the major school science equipment suppliers. Sterilization is at 121 °C (15 lb in ˉ²) for 15 minutes. pH values are 7.0 unless stated otherwise.
Note: Allow 15 cm³ of agar for each Petri dish and 5-10 cm³ of broth for each McCartney bottle. All cotton wool plugs should be made of non-absorbent cotton wool. Plastic or metal caps may also be used.
Nutrient agar
Suspend 28 g of nutrient agar powder in 1 litre of distilled water. Bring to the boil to dissolve completely. Dispense as required and sterilize.
Nutrient broth
Add 13 g of nutrient broth powder to 1 litre of distilled water. Mix well. Dispense as required and sterilize.
Malt extract agar
Suspend 18g agar powder in 1 litre of distilled water. Bring to the boil to dissolve completely. Add 15g malt extract per litre. Mix well. Dispense as required and sterilize.
Mannitol yeast extract agar
Suspend 10 g agar in 1 litre of distilled water. Heat to dissolve. Add 0.5 g K2HPO4 , 0.2g MgSO4.7H2O, 0.2 g NaCl, 0.2 g CaCl2.6H2O, 10 g mannitol and 0.4 g yeast extract. Dispense as required and sterilize.
Mannitol yeast extract broth
As above, without agar.
Glucose nutrient broth
Make up nutrient broth as already directed and add 10 g per litre of glucose.
Sugar peptone water
Add 10 g of peptone, 5 g of NaCl, 5 g of sugar and 20 cm³ of Universal indicator to 1 litre of distilled water; pH should be 7.4. Dispense as required and sterilize.
Tributyrin agar
Supplied ready for use. Heat to melt and dispense aseptically. May be prepared by adding 1% tributyrin to nutrient agar.
Glucose yeast extract broth
Add 10 g of peptone, 5 g of NaCl, 3 g of yeast extract to 1 litre of distilled water. Dispense as required and sterilize.
Glucose yeast extract lemco broth
Add 10 g of Lemco (meat extract) to glucose yeast extract broth.
Milk agar
Make up nutrient agar as above but using only 900 cm³ of distilled water. Dissolve 20 g of dried skimmed milk in 100 cm³ of distilled water. Sterilize separately. Transfer the milk to the agar aseptically after cooling to 45-50 °C. Dispense aseptically.
Starch agar
Suspend 15 g of nutrient agar in 100 cm³ distilled water. Bring to the boil to dissolve completely. Heat 40 g of soluble starch in 100 cm³ of distilled water to form a suspension. Allow to cool and then mix with the nutrient agar solution. Dispense and sterilize.
Iodine solution
Dissolve 1 g of iodine crystals and 2 g of potassium iodine in 300 cm³ of distilled water.
Cellulose broth (for Trichoderma reesei)
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800 cm3 distilled water
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0.1 g CaCl2
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0.5 g (NH4)2SO4
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0.5 g yeast extract powder
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0.5 g asparagine
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10 g carboxymethylcellulose
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1.0 g KH2PO4
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pH6.2
Mix ingredients, heat gently, and stir until dissolved.
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