Cetrimide Agar, also known as Pseudomonas Cetrimide Agar or Pseudosel Agar, is a selective and differential medium used for the isolation and identification of Pseudomonas aeruginosa from clinical and non-clinical specimens.
Cetyltrimethylammonium bromide (Cetrimide) is a mixture of different quaternary ammonium salts that serve as the selective agent of this media. The mixture, acting as a detergent, causes a release of nitrogen and phosphorus from the bacteria cell, denaturing the cell membrane for bacteria except for P. aeruginosa. Other species of Pseudomonas are still susceptible to this compound. Cetrimide also serves as way to detect an organisms ability to produce fluorescent pigments.
Pancreatic digest of gelatin provides nitrogen, carbon, and other vitamins viable for the growth of P. aeruginosa. Glycerol serves as a supplemental carbon source and addition of magnesium chloride and potassium sulphate stimulate the production of pyocyanin and pyoverdin. Agar is the solidifying agent.
Pseudomonas aeruginosa is an opportunistic aerobic, gram-negative bacterium found widely throughout nature and is one of the most commercially and biotechnological valuable microorganisms.
P. aeruginosa produces soluble iron pigments: pyocyanin, pyoverdin, pyomelanin, and pyorubrin. No other species of gram-negative, non-fermenting bacteria produces pyocyanin which is a key element in the selectivity of the agar. When pyoverdin combines with pyocyanin, a bright green phenazine pigment is produced. Cetrimide, potassium sulfate, and magnesium chloride enhance and stimulate the production of both pyocanin and pyoverdin.
On the selective agar, the presence of growth indicates a positive reaction. Examination of individual colonies under ultraviolet light (254 nm) allows for detection of fluorescent pigments.
PREPARATION OF CENTRIMIDE AGAR
Add both dry and wet components to distilled water and boil to dissolve completely.
Sterilize media by autoclaving at 121ºC for 15 minutes.
Cool sterilized media to 45-50ºC.
Aseptically dispense into sterile petri dishes and/or other appropriate containers.
Final pH: 7.2 ± 0.2 at 25°C
STORAGE AND SHELF LIFE
Media is both light and temperature sensitive. Store plates away from direct light at 2-8ºC, or if properly sealed and stored upright, 15-25ºC. Plates may be used for one week when stored in a clean sterile area. Media should not be used if any signs of deterioration, color change, contamination, and/or expiration date has passed.
STERILIZE ALL BIOHAZARD WASTE BEFORE DISPOSAL.
After 18-24 hours of incubation (or ≥ 72 hours for negative controls) in a 30-35°C, plates should show isolated colonies in streaked areas, and confluent growth/lawns in areas of heavy inoculation. Colonies may be identified as P. aeruginosa when exhibiting a blue/green pigment fluorescing under short wavelength ultraviolet light. (254 nm)
Note: Certain strains of P. aeruginosa may not produce pyocyanin. Other species of Pseudomonas do not produce pyocyanin, but will fluoresce under UV light. Some non-fermenting aerobic spore formers may exhibit a water-soluble tan to brown pigmentation on the medium. Serratia spp. may exhibit pink pigmentation.
Suitable microorganisms for Quality Control:
Slight yellowing of the medium may occur due to enteric strains and/or environmental variables, however coloration can be distinguished using UV light.
P. aeruginosa can lose its fluorescence under ultraviolet light if cultures are left at room temperature. Fluorescence reappears when re-incubated.
Further tests may be necessary for confirmation of P. aeruginosa.
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