PCA agar | Plate count agar

PCA agar | Principle | Preparation | Interpretation

☰ Content :

Ⅰ. Overview

PCA agar (Plate Count Agar) is a medium recommended for the standardized enumeration of aerobic bacteria in water, dairy products and foods, cosmetics or pharmaceuticals. PCA medium is non-selective and relatively rich in nutrients, tryptone, vitamin factors from yeast extract and glucose used as an energy source promote the growth of most bacteria.

> This medium, also called Tryptone Glucose Yeast Agar or Casein-Peptone Dextrose Yeast Agar, meets the specifications given by the American Public Health Association and ISO 4833.

PCA agar

Ⅱ. Preparation / Composition

Suspend the components, dehydrated powder, in water (23.5 in 1000 ml of purified/distilled water). The medium is boiled for a few seconds until the ingredients are completely dissolved.
Sterilize in an autoclave at 121°C for 15 minutes. Pour into sterile Petri dishes and let solidify on a flat surface

Composition PCA agar
Ingredients	gram/liter
Tryptone	5g
Yeast extract	2.5g
Glucose	1g
Bacteriological agar agar	12g
pH	at 25°C: 7.0 ± 0.2.

◈ In dairy bacteriology, it is recommended to add 1 g of skimmed milk powder per liter of reconstituted medium

Ⅲ. Principle and use

The amount of bacteria is expressed in colony forming units per gram (CFU/g), in solid samples and per ml (CFU/ml) in liquid samples. This value can be determined by several techniques:

Plate Count Agar (PCA), also called Standard Methods Agar (SMA)

Transfer 1 mL of the inoculum and its successive decimal dilutions into sterile Petri dishes.
Pour approximately 15 mL of medium brought to 44-47°C, per dish.
Homogenize perfectly and leave to solidify on a cold surface.
Incubate at 30 ± 1°C for 72 ± 3 hours.

NOTE : If the presence of colonies invading the surface of the agar is suspected, it is possible to pour a second layer of agar, after taking up the first mass (approximately 4 mL).

Spread plate Method

Dry the boxes in the oven with the lid ajar.
At the surface of the medium, transfer 0.1 mL of the sample to be analyzed and its successive decimal dilutions.
Inoculate the inoculum on the surface using a sterile spreader.
Incubate at 30 ± 1°C for 72 ± 3 hours.

Ⅳ. Interpretation

◈ Incubate:
- at 30°C for 72 hours for the detection of mesophilic microorganisms.
- at 55°C for thermophilic microorganisms.
- at 6.5°C for psychrotrophic microorganisms for 10

After incubation, count the colonies on the dishes containing between 10 and 300 colonies. When calculating the actual number of bacteria in the sample, the dilution factor must be taken into account.

When cultured on PCA agar supplemented with milk, caseolytic bacteria form a lighter halo around each colony (milk casein proteolysis)

Bacteria	Growth
Staphylococcus aureus	Good growth, productivity P_R ≥ 0.70
Escherichia coli	Good growth, productivity P_R ≥ 0.70
Bacillus subtilis	Good growth, productivity P_R ≥ 0.70

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