PCA, also known as plate count (PCA) is an bacteriological medium that is used to determine the total amount of aerobic live bacteria present in a sample. This is not considered to be a specific medium. The quantity of bacteria is expressed in units of colony-forming units per Gram (CFU/g) in solid samples, and per milliliter (CFU/ml) for liquid samples. The preferred method is to use the pour plate method. The samples are dilute and the appropriate dilutions added to Petri plates. Sterile molten Agar is added to the plates. The plates are rotated with care to ensure an even mixing of the sample with the agar. The plates are then incubated at 20 or 30degC over three days. After incubation, the amount of colonies counted is recorded on the plate using 25-250 colonies, which is believed to provide the most precise results. When calculating the amount of bacteria present within the specimen, the dilution factors is to be considered.
Principle of Plate Count Agar (PCA)
Plate Count Agar can also be known as Tryptone Glucose-Yeast Agar or Casein-Peptone Dextrose Agar. The medium is an enzymatic digest made of casein which provides amino acids and nitrogen carbon, vitamins and minerals that aid in the growth of the organism. The primary ingredient in yeast extract is B-complex vitamins. The carbohydrate glucose is fermentable and serves as an energy source to help the development of bacteria. Agar is the substance that makes a solid.
Composition of Plate Count Agar (PCA)
| Ingredients | Gms/L |
| Enzymatic Digest of Casein/tryptone | 5.0 |
| Yeast Extract | 2.5 |
| Glucose | 1.0 |
| Agar | 15.0 |
Final pH 7.0 ± 0.2 at 25°C
Preparation of Plate Count Agar (PCA)
- Take 23.5 grams in 1000ml distillate water.
- Bring the temperature to a boil until the medium is completely dissolving.
- Sterilize with autoclaving with 15lbs tension (121degC) over 15 mins.
- Cool to 45-50degC.
- Mix well before pouring into sterilized Petri plates.
Results on Plate Count Agar (PCA)
Collect colonies in all plates with 15 to 300 colonies. Record the count in CFU/ml in the liquid sample or CFU/g when using the solid sample, allowing for Dilution Factors.
Bacillus subtilis = Growth straw-colored colonies
Escherichia Coli = Growth straw-colored colonies
Staphylococcus aureus = Growing straw-colored colonies
Uses of Plate Count Agar (PCA)
- Plate Count Agar is utilized for the measurement of the amount of bacteria present in food, water, and other materials important to sanitary.
- It can also be used for counting the number of bacteria in the sterile rooms.
Limitations
- It is suggested that biochemical, immunological mass spectrometry or molecular tests be carried out on the colonies of pure culture to confirm the identity of the colony.
- The Plate Count Agar medium is general use medium, and it is not able to allow the growth of fastidious organisms.
