What is a Cetrimide Agar?
Cetrimide, a quaternary salt of ammonium, acts as a detergent that lowers the surface tension at the point-of-contact. It also has precipitant, complexing, and denaturing effects upon bacterial membrane proteins. It has inhibitory properties on many microorganisms, including Pseudomonas species that are not Pseudomonas. Lowburry was the first to develop cetrimide agar. It is a modified version of Tech Agar (developed in King et al. For the selective inhibition other than Pseudomonas, aeruginosa, 0.1% cetrimide (cetyltrimethyl ammonium bromide), was added. Cetrimide agar can be used to presumptive identify and selectively isolate Pseudomonas.aeruginosa species from both clinical and nonclinical specimens.
Principle of Cetrimide Agar
Cetrimide agar can be used to test the ability of an organism grow in the presence or absence of cetrimide. Cetrimide is a toxic substance that inhibits many bacteria’s growth and causes the release of nitrogen, phosphorous, which slows down or kills them. P. aeruginosa, however, is immune to cetrimide. It is therefore used to isolate Pseudomonas. aeruginosa in clinical specimens. Pseudomonas is aeruginosa are easily identified by their distinctive production of pyocyanin (a blue, non-fluorescent, water-soluble phenazine pigment), along with their colonial structure and characteristic grape-like aroma of aminoacetophenone. P. aeruginosa, a gram-negative rod, is the only known species of Pseudomonas that excretes pyocyanin.
Cetrimide, a quaternary sodium salt, acts as a detergent when it comes into contact with bacterial cells. This causes the release of nitrogen, phosphorous, and in turn denaturing the membrane proteins of the bacteria cell. Pseudomonas (fluorescein) produces many water-soluble pigments. The bright green color of Pseudomonas is created when pyoverdin and the blue-water-soluble pigment, pyocyanin. Cetrimide increases the production of both fluorescein and pyocyanin. P. aeruginosa is provided with the necessary nutrients by gelatin peptone. The medium is maintained in an osmotic equilibrium by sodium chloride. Magnesium chloride, potassium sulfate, and magnesium chloride stimulate the production of pyocyanin, a blue-green pigment that diffuses into the medium. Glycerol is the carbon source. Agar acts as the solidifying agent.
Composition of Cetrimide Agar
| Ingredients | Grams/liter |
| Gelatin peptone | 20.0gm |
| Magnesium chloride | 1.4gm |
| Potassium sulfate | 10.0gm |
| Cetrimide | 0.3gm |
| Glycerol | 10.0ml |
| Agar | 13.6gm |
Final pH (at 25°C) 7.2±0.2
Preparation of Cetrimide Agar
- In 1 litre of distilled, add 45.3 grams of the medium.
- To dissolve the glycerol, add 10ml to the boiling water and boil.
- For 15 minutes, sterilize by autoclaving at 120°C
- Allow the medium to cool to 50°C, then pour it into sterilized Petri dishes.
Result Interpretation of Cetrimide Agar
| Results | Observations |
| Positive | Pigmented blue, blue-green colonies.Colonies exhibiting fluorescence at 250nm and a blue-green pigmentation are considered as presumptive positive. |
| Negative | No growth |
Uses of Cetrimide Agar
- It is used primarily for the presumptive identification and selective isolation of Pseudomonas Aeruginosa from clinical specimens.
- It can also be used to determine if an organism is capable of producing fluorescein or pyocyanin.
- It is used for the isolation and purification of Pseudomonas.aeruginosa (contaminated specimens).
Limitations of Cetrimide Agar
- Cetrimide is toxic and this medium can be selective. Some strains may not grow well in this medium.
- Some enterics may show a slight yellowing in the medium from time to time. However, this is not fluorescein production and can be easily distinguished from fluorescein production.
- This medium may show a water-soluble brown to tan pigmentation in some non-fermenters as well as some aerobic spores. Pink pigmentation may be seen in Serratia strains.
- Collins and Lowbury’s studies showed that P. aeruginosa fluorescence can be lost under UV exposure if cultures are kept at room temperature for too long. Fluorescence returns when plates are re-incubated.
- To confirm the results and confirm the diagnosis of P. Aeruginosa, additional testing should be done such as biochemical and serological tests.
