Cetrimide agar

Cetrimide Agar

Cetrimide agar is a type of growth medium used in microbiology for the selective isolation and identification of Pseudomonas aeruginosa, a common Gram-negative bacterium. This medium is particularly useful in clinical and environmental microbiology for detecting this pathogen in various samples.

Composition

Cetrimide agar contains several key components that make it selective for Pseudomonas aeruginosa:

• Cetrimide: A quaternary ammonium compound that acts as a selective agent by inhibiting the growth of most other bacteria, allowing Pseudomonas aeruginosa to thrive.
• Magnesium chloride and potassium sulfate: These salts enhance the production of pyocyanin, a blue-green pigment characteristic of Pseudomonas aeruginosa.
• Peptone: Provides essential nutrients for bacterial growth.
• Agar: A solidifying agent that provides a surface for bacterial colonies to grow.

Mechanism of Action

Cetrimide agar works by exploiting the natural resistance of Pseudomonas aeruginosa to cetrimide. The medium's high salt concentration and the presence of cetrimide inhibit the growth of most other bacteria, while Pseudomonas aeruginosa can grow and produce its characteristic pigments, such as pyocyanin and fluorescein.

Uses

Cetrimide agar is primarily used in:

• Clinical microbiology: For the isolation of Pseudomonas aeruginosa from clinical specimens such as sputum, urine, and wound swabs.
• Environmental microbiology: To detect Pseudomonas aeruginosa in water samples and other environmental sources.
• Pharmaceutical microbiology: To ensure the absence of Pseudomonas aeruginosa in non-sterile pharmaceutical products.

Procedure

1. Prepare the cetrimide agar according to the manufacturer's instructions. 2. Sterilize the medium by autoclaving. 3. Pour the medium into sterile Petri dishes and allow it to solidify. 4. Inoculate the sample onto the surface of the agar. 5. Incubate the plates at 35-37°C for 24-48 hours. 6. Examine the plates for the presence of characteristic colonies of Pseudomonas aeruginosa, which appear as blue-green or yellow-green due to pigment production.

Limitations

While cetrimide agar is effective for isolating Pseudomonas aeruginosa, it is not entirely specific. Some other Pseudomonas species and non-Pseudomonas bacteria may also grow on this medium. Therefore, further biochemical tests are often required to confirm the identity of the organism.

Also see

• Pseudomonas aeruginosa
• Microbiological culture
• Selective media
• Pyocyanin