Plate Count Agar is a medium used for the determination of the total microbial content from food and animal feed, water and other materials.

Method Principal

Enzymatic digest of casein provides amino acids, nitrogen, carbon, vitamins and minerals for organisms growth. Yeast extract is a source of vitamins, particularly of B-group. Glucose is the fermentable carbohydrate. Agar is the solidifying agent.

Preperation

Dehydrated medium: Suspend 23.5 g of the powder in 1 liter of distilled or deionized water. Mix well. Heat to boil shaking frequently until completely dissolved. Sterilize in autoclave at

121°C for 15 minutes.

Note: ISO 4833 recommends to add 1.0 g of skimmed milk powder per liter of

medium when dairy products are examined.

Medium in tubes/bottles: Melt the content of the tube/bottle in a water bath at 100°C (loosing the cap partially removed) until completely dissolved. Then screw the cap and check the homogeneity of the dissolved medium, if it is the case turning the tube/bottle upside down. Cool at 45-50°C, mix well avoiding foam formation and aseptically distribute into Petri dishes.

Test Procedure

1. Perform serial dilutions of the test sample in order to achieve a colony count of between 15 and 300 colonies per plate. Use a suitable diluent such as Buffered Peptone Water (ref. 24099) or Maximum Recovery Broth (ref. 20071).

2. Inoculate the medium by pour plating, spread plating or membrane filtration method.

3. Incubation conditions may vary depending on the organisms under study. For a general aerobic count, incubate aerobically at 30°C for 72 hours.

Interpreting Results

Count colonies on all plates containing 15-300 colonies. Report the count as CFU per ml of sample allowing for dilution factors.

Apperance

Dehydrated medium: free-flowing, homogeneous, light beige.

Prepared medium: slightly opalescent, light amber.

Storage

The powder is very hygroscopic, store the powder at 10-30°C, in a dry environment, in its original container tightly closed. Store bottles, tubes and prepared plates at 10-25°C away from light. Do not use the product beyond its expiry date on the label or if product shows any evidence of contamination or any sign of deterioration.

Shelf Life

Dehydrated medium: 4 years.

Medium in tubes/bottles: 2 years.

Medium in slant tubes: 1 year.

Ready-to-use plates: 6 months.