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Dehydrated Culture Media

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YEAST AND MOULD AGAR

Code: CM0920

A medium for the isolation of yeasts and moulds.

Typical Formula*

gm/litre

Yeast extract

3.0

Malt extract

3.0

Peptone

5.0

Dextrose

10.0

Agar

20.0

Final pH 6.2 ± 0.2 @ 25°C

 

* Adjusted as required to meet performance standards 

Directions
Suspend 41.0g in 1 litre of distilled water and bring to the boil to dissolve. Sterilise by autoclaving at 121°C for 15 minutes. The medium may be rendered selective after sterilisation by acidifying to pH 4.0 with 12-15ml of Lactic Acid SR0021 after cooling to 50°C. Do not reheat after making this addition. Mix well and pour into sterile Petri dishes.

Description
Yeast and Mould Agar is based on the formulation described by Wickerham1,2. The medium is recommended for the isolation and maintenance of yeasts and moulds.

Yeast and Mould Agar may be rendered selective by the addition of acid to reduce the pH of the medium to pH 4.0. A suitable acid for use is Lactic Acid SR0021.

Yeast and Mould Agar has now been formulated specifically for suitability within the Brewing Industry, for detection of both saccharomyces and non saccharomyces wild yeasts in the presence of culture yeast.

The use of MYGP + Copper Medium was described by Taylor and Marsh for selective isolation of wild yeasts in the presence of culture ale or larger yeasts3.

The medium utilises the the different sensitivities of wild yeast vs.culture ale or lager yeast to the inhibitory action of copper. This enables wild yeasts to grow on the medium whilst suppressing growth of culture yeast.

The authors recognised that variation in raw materials, including peptones, malt and yeast extract and agar, in the formulation could markedly affect the performance of the medium by neutralising the inhibtory action of the copper, resulting in overgrowth of the wild yeasts by culture yeast.
Careful selection of the raw materials by Oxoid has eliminated this variability, and the medium has been optimised for addition of copper, usually within the range 50-300 mg/litre depending on the sensitivity of the culture yeast to copper, as recommended by the Institute of Brewing Methods of analysis4.

The complete medium is recommended for examination of the microbiological quality of beers in process, pitching yeasts and packaged beers.

Technique
1. Prepare Yeast and Mould Agar plates as directed in the directions for use.
2. Inoculate the medium by surface or poured plate procedures.
3. Incubate the plates for 48-72 hours at 25-30°C.

Detection and enumeration of yeasts in the presence of moulds may be made easier by using a combined anaerobic/aerobic incubation procedure5.

Cultures are initially incubated for 3 days under anaerobic conditions and then for a further 2 days aerobically. Development of mould colonies is impeded during the anaerobic phase of incubation. Dimorphic moulds e.g. mucor spp may form yeast- like colonies during anaerobic incubation.

Technique
Cultures on plates or membrane filters are incubated for 3 days at 25°C under strictly anaerobic conditions. Continue incubation of the cultures under aerobic conditions for a further 2 days. Yeast colonies may be very small immediately following anaerobic incubation but will increase in size in air. Mould growth may become completely unrestricted after 3 days in air.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
The prepared medium may be stored for up to 2 weeks at 2-8°C.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Straw coloured gel

Quality control

Positive controls:

Expected results

 

pH 6.2

pH 4.0

Aspergillus brasiliensis ATCC® 16404 *

Good growth; white mycelium, black spores

Good growth; white mycelium, black spores

Candida albicans ATCC® 10231 *

Good growth; cream coloured colonies

Good growth; cream coloured colonies

Eschericia coli ATCC® 25922 *

Good growth; straw coloured colonies

Partial to complete inhibition

* This organism is available as a Culti-Loop®

Precautions
For in vitro diagnostic use.
Do not use beyond the stated expiry date or if the product is caked, discoloured or shows any sign of deterioration.

References
1. Wickerham L.J. (1951) U.S. Dept. Agric. Tech. Bull. No 1029, 1-19.
2. Wickerham L.J. and Rettger L.F (1939) J. Tropical Med. Hyg. 42. 174-179.
3. Taylor, G.T. and Marsh, A.S. (1984) J. Inst. Brew., 90: 134-145.
4. Institute of Brewing Methods of Analysis. 1997 Vol. 2 Microbiological. 23.45
5. De Jong J. and Put H.M.C. (1980) Biology and Activities of Yeasts. Society for Applied Bacteriology Symposium series No. 9. Skinner F.A., Passmore S.M. and Davenport R.R. (eds). Academic Press, London. Pages 289-292.

 
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