Cetrimide Agar is a selective and differential medium used for the isolation and identification of Pseudomonas aeruginosa from clinical and non-clinical specimens. Cetrimide is the selective agent and inhibits most bacteria by acting as a detergent ( Cetyltrimethylammonium bromide, a quaternary ammonium, cationic detergent).
It is also known as Pseudomonas Cetrimide Agar or Pseudosel Agar.
Composition of Cetrimide Agar
|Pancreatic Digest of Gelatin||
Final pH 7.2 +/- 0.2 at 25 degrees C
Principle of Cetrimide Agar
Cetrimide Agar is used as a selective medium for the isolation of Pseudomonas aeruginosa from pus, sputum and
drains, etc. Pseudomonas aeruginosa produces a number of water soluble iron chelators, including the yellow-green or yellow-brown fluorescent pyoverdin. When pyoverdin combines with the blue water-soluble pyocyanin, the bright green colour characteristic of Pseudomonas aeruginosa is created.
Pancreatic digest of gelatin provide necessary nutrients for P. aeruginosa such as nitrogen, vitamins, and carbon.
The addition of magnesium chloride and potassium sulphate stimulates pyocyanin and pyoverdin (fluorescein) production.
Cetyltrimethylammonium bromide (Cetrimide) is the selective agent and inhibits most bacteria by acting as a detergent. When in contact with bacteria, causes the release of nitrogen and phosphorous from the bacterial cell other than Pseudomonas aeruginosa.
Glycerol is supplemented as a source of carbon.
Agar is the solidifying agent.
Uses of Cetrimide Agar
- It is primarily used for the selective isolation and presumptive identification of Pseudomonas aeruginosa
from clinical and nonclinical specimens.
- It is also used for determining the ability of an organism to produce fluorescein and pyocyanin (Antibiotica)
Preparation of Cetrimide Agar
- Add 45.3 gm of the medium in 1 litre of distilled water.
- Add 10ml of glycerol and boil to dissolve completely.
- Sterilize by autoclaving at 121°C for 15 minutes.
- Cool the medium to approximately 50°C and pour into sterile Petri dishes.
Interpretation of Results on Cetrimide Agar
The presence of growth is indicative of a positive reaction. Examine colonies under short wavelength (254nm) ultraviolet light for the presence of fluorescein. Visual examination may also reveal the typical yellow-green to blue color which indicates the production of pyocyanin. Both pyocyanin and fluorescein are typically produced by strains of P. aeruginosa.
A negative reaction is denoted by no growth.
Colony Morphology on Cetrimide Agar
Quality Control on Cetrimide Agar
Pseudomonas aeruginosa ATCC 9027 – Yellow-green to blue colonies.
Escherichia coli ATCC 8739 – Partial to complete inhibition. No Pigmentations.
Limitation of Cetrimide Agar
- Occasionally some enterics will exhibit a slight yellowing of the medium; however, this coloration is easily
distinguished from fluorescein production because this yellowing does not fluoresce.
- Some non-fermenters and some aerobic spores formers may exhibit a water-soluble tan to brown pigmentation on this medium. Serratia strains may exhibit a pink pigmentation.
- Studies of Lowbury and Collins showed P. aeruginosa can lose its fluorescence under UV if the cultures are left at room temperature for a short time. Fluorescence reappears when plates are re-incubated.
- Further tests are necessary for confirmation of P. aeruginosa.