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Cetrimide Agar: Introduction, Principle, Composition, Preparation, Procedure, Colony Morphology, Uses and Keynotes

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Cetrimide Agar Introduction, Principle, Composition, Preparation, Procedure, Colony Morphology, Uses and Keynotes

Cetrimide Agar Introduction, Principle, Composition, Preparation, Procedure, Colony Morphology, Uses and Keynotes

Introduction of Cetrimide Agar

Cetrimide agar is a selective medium used for the isolation of Pseudomonas aeruginosa.

Principle of Cetrimide Agar

Cetrimide of the medium inhibits bacteria other than Pseudomonas aeruginosa. It is a cationic detergent that acts as a quaternary ammonium compound, which causes nitrogen and phosphorus to be released from cells of bacteria other than P. aeruginosa . Magnesium chloride and potassium sulfate in the medium enhance the production of the pigment pyocyanin, which is a blue-green pigment, diffusing into the medium. This helps the detection of Pseudomonas on this medium. The presence of magnesium ions can also neutralize anticoagulant, EDTA if present in the sample. The pancreatic digest of gelatin gives the essential nutrients for the growth of Pseudomonas, while glycerin serves as a slow and continuous carbon source for the growing bacterial cells.

Composition of Cetrimide Agar

Ingredients    Gms / Litre
Pancreatic digest of gelatin:  20.0
Magnesium chloride: 1.40
Dipotassium sulfate: 10.0
Cetrimide:  0.3
Agar: 13.6
pH after sterilization ( at 25°C) 7.2±0.2

 Preparation of Cetrimide Agar

  1. Suspend 4.53 grams in 100 ml purified/distilled or deionized water.
  2. Add 1 ml glycerol.
  3. Heat to boiling to dissolve the medium completely.
  4. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
  5. After autoclaving,  leave for cooling to 45-50°C.
  6. Pour agar into each plate and leave plates on the sterile surface until the agar has solidified.
  7. Store the plates in a refrigerator at 2-8°C.

Storage and Shelf life 

Test Requirements

Procedure of Cetrimide Agar (Sample inoculation/ organisms inoculation)

  1. Allow the plates to warm at 37°C or to room temperature, and then leave the agar surface to dry before inoculating.
  2. Inoculate and streak the specimen as soon as possible after collection.
  3. If the specimen is on a swab, roll the swab over a small area of the agar surface.
  4. Streak for isolation with a sterile loop.
  5. Incubate plates aerobically at 30-35ºC for 18-24 hours.
  6. Examine colonial characteristics.

Result -Interpretation of Cetrimide Agar

Colony Morphology of Cetrimide Agar

Pseudomonas aeruginosa: Pigmented greenish

Uses of Cetrimide Agar

It is used for the isolation of P. aeruginosa mainly from pharmaceutical products in accordance with microbial limit testing.

Keynotes on Cetrimide Agar

  1. The color and Clarity of the prepared medium is a light amber-colored opalescent gel with a slight precipitate formed in Petri plates.
  2. The addition of nalidixic acid in cetrimide agar can aid in inhibiting the growth of accompanying flora.
  3. If the bacterial load (P. aeruginosa) is low in the sample, Soyabean Casein Digest Medium (non-selective medium) is used for primary inoculation and later subcultured into cetrimide agar.
  4. This agar is also used for microbial limit testing for non-sterile products.
  5. Cetrimide is an N-acetyl-N, N, N-trimethylammonium bromide.
  6. It is a selective agent for the isolation of Pseudomonas initially reported by Lowburry.

Further Readings on Cetrimide Agar