YEAST EXTRACT AGAR

Code: CM0019

A nutrient agar for the plate count of organisms in water.

Description
This medium is made to the formula described by Windle Taylor¹ for the plate count of micro-organisms in water.

In the UK the usual method of counting heterotrophic bacteria in water is by the poured plate method with Yeast Extract Agar. Separate counts are made of those aerobic mesophilic organisms which form visible colonies in this medium after 24 hours incubation at 37°C and those which form colonies after 3 days incubation at 22°C. The two methods give different results².

The organisms growing under these conditions comprise bacteria, yeasts and moulds.

The Quality Control of Yeast Extract Agar CM0019 includes testing in accordance with ISO 11133:2014³.

Technique
1. Prepare appropriate decimal dilutions of the water sample (for example Quarter Strength Ringer’s 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 48 hours at 37°C and 3 days at 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.

The medium can also be used as a spread plate. Refer to the appropriate local, national or international standard for the exact method to follow.

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.