Cetrimide Medium

Cetrimide media are used as a selective medium for the isolation of Pseudomonas aeruginosa and for microbial limit testing for non-sterile products. Lowburry first reported the use of cetrimide as an agent for selective isolation of Pseudomonas1. Additionally this medium is also used for determining the ability of an organism to produce fluorescein and pyocyanin. Cetrimide, a quaternary ammonium detergent, is incorporated in the medium to inhibit the growth of bacteria other than Pseudomonas aeruginosa. Magnesium chloride and potassium sulphate enhances the production of blue-green pigment pyocyanin which diffusing into the medium after production improving the detection of Pseudomonas.

For the isolation of Pseudomonas aeruginosa, plates of Cetrimide Agar should be inoculated from non-selective medium such as Soyabean Casein Digest. If the count is high the test sample can be directly inoculated onto this medium.

Quality Control

The following organisms are routinely used for testing for this medium.
Test OrganismsResults
Pseudomonas aeruginosa ATCC® 27853 Growth; yellow-green to blue colonies
Pseudomonas aeruginosa ATCC® 9027 Growth; yellow-green to blue colonies
Escherichia coli ATCC® 25922 Partial to complete inhibition
Escherichia coli ATCC® 8739 Partial to complete inhibition
Staphylococcus aureus ATCC® 25923 Partial to complete inhibition

User Quality Control

Check for signs of contamination and deterioration. Users of commercially prepared media may be required to perform quality control testing with at least one known organism to demonstrate growth or a positive reaction; and at least one organism to demonstrate inhibition or a negative reaction (where applicable).

Ingredients g/L

(Final pH 7.2 +/- 0.2 at 25°C)
Pancreatic Digest of Gelatin 20.0
Potassium Sulfate 10.0
Magnesium Chloride 1.4
Cetyltrimethylammonium Bromide 0.3
Agar 13.6
1 total citations

  1. Lowbury E J L., 1951, J.Clin.Path., 4:66.
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