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Dehydrated Culture Media

WL NUTRIENT AGAR (MEDIUM)

Code: CM0309

A medium for the determination of the microbiological flora in brewing and fermentation which can be made selective for bacteria with cycloheximide.

Typical Formula*

gm/litre

Yeast extract

4.0

Tryptone

5.0

Glucose

50.0

Potassium dihydrogen phosphate

0.55

Potassium chloride

0.425

Calcium chloride

0.125

Magnesium sulphate

0.125

Ferric chloride

0.0025

Manganese sulphate

0.0025

Bromocresol green

0.022

Agar

15.0

pH 5.5 ± 0.2

 
* Adjusted as required to meet performance standards

Directions
Suspend 75g in 1 litre of distilled water. Bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes. If required the pH may be adjusted to 6.5 by the addition of 1% sodium bicarbonate solution.

Description
WL Nutrient Agar (Medium), based on that of Green and Gray1 is recommended for the determination of the microbiological flora in brewing and fermentation processes. The medium is suitable for the differentiation of `wild’ yeasts from brewing yeasts2.

Reliable counts for brewers’ yeast are obtained with the medium at pH 5.5. Adjustment to pH 6.5 facilitates the counting of bakers’ and distillers’ yeasts.

Cycloheximide (SR0222) may be added to the medium at a final concentration of 4 mg/litre, to suppress yeast growth, the medium becomes selective for the bacterial contaminants of yeast cultures.

When making microbial counts with this medium the time and temperature of incubation will vary according to the materials tested and the organisms sought. Temperatures of 25°C are used for brewing materials and 30°C for bakers’ yeasts.

Incubation
Times can vary from 2 to 14 days. Aerobic or anaerobic incubation conditions will depend on the characteristics of the organisms.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared medium at 2-8°C.

Appearance
Dehydrated medium: White or pale straw coloured, free-flowing powder
Prepared medium: Blue coloured gel

Quality control

w/o cycloheximide

 

Positive control:

Expected results

Saccharomyces cerevisiae ATCC® 9763 *

Good growth; green colonies with dark green centres

Negative control:

 

Uninoculated medium

No change

with cycloheximide  
Positive control:  
Escherichia coli ATCC® 25922 *Good growth; green coloured colonies

Negative control:

 
Saccharomyces cerevisiae ATCC® 9763 *No growth
* This organism is available as a Culti-Loop®

Precautions
When handling cycloheximide observe the precautions to be taken under HAZARDS.

References
1. Green S. R. and Gray P. P. (1950) Wallerstein Lab. Comm. 13. 357.
2. Hall Jean F. (1971) J. Inst. Brewing 77. 513-516.

 
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