YEAST EXTRACT AGAR
Code: CM0019
A nutrient agar for the plate count of organisms in water.
Typical Formula* | gm/litre |
Yeast extract | 3.0 |
Peptone | 5.0 |
Agar | 15.0 |
pH 7.2 ± 0.2 @ 25°C | |
* Adjusted as required to meet performance standards
Directions
Suspend 23g in 1 litre of distilled water. Bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes.
Description
This medium is made to the formula described by Windle Taylor for the plate count of micro-organisms in water1.
In the UK the usual method of counting heterotrophic bacteria in water is by the poured plate method with Yeast Extract Agar. Seperate counts are made of those aerobic mesophilic organisms which form visible colonies in this medium after 24 hours incubation at 35°C and those which form colonies after 3 days at 20-22°C. The two methods give different results2.
The organisms growing under these conditions comprise bacteria, yeasts and moulds.
Technique
1. Prepare appropriate decimal dilutions of the water sample (with Ringer Solution) and pipette 1ml portions of the water and each dilution into duplicate sterile petri dishes.
2. Add 15ml of Yeast Extract Agar (previously melted and cooled to 45-50°C) to each plate. Mix the contents by a combination of rapid to-and-fro shaking and circular movements lasting over a period of 5-10 seconds.
3. Allow to solidify, and incubate duplicate sets of plates for 24 hours at 37°C and 3 days at 20-22°C respectively.
4. Select plates containing 30-300 colonies for counting. No count should be made on a plate containing less than 30 colonies unless the plates from the undiluted water contain less than this number.
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: Light straw coloured, free-flowing powder
Prepared medium: Straw coloured gel
Quality control
Positive control: | Expected results |
| Escherichia coli ATCC® 25922 * | Good growth; cream coloured colonies. |
Negative control: | |
Uninoculated medium. | No change |