Cetrimide agar

Cetrimide agar is a selective growth medium used in microbiology for the isolation of Pseudomonas aeruginosa, a common Gram-negative, opportunistic pathogen. This medium is particularly useful in clinical and environmental microbiology for identifying and studying this bacterium due to its ability to inhibit the growth of other microorganisms while promoting the growth of P. aeruginosa.

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.
• Magnesium chloride and potassium sulfate: These salts enhance the production of pyocyanin and fluorescein, pigments characteristic of P. aeruginosa.
• Agar: A solidifying agent that provides a stable medium for bacterial growth.
• Peptone: A source of nitrogen and other nutrients for bacterial growth.

Mechanism of Action

Cetrimide acts by disrupting the cell membranes of bacteria, which inhibits the growth of most Gram-positive and many Gram-negative bacteria. However, Pseudomonas aeruginosa is resistant to cetrimide due to its robust cell wall and efflux pump mechanisms, allowing it to thrive on this medium.

Identification of Pseudomonas aeruginosa

On cetrimide agar, Pseudomonas aeruginosa can be identified by its characteristic colony morphology and pigment production:

• Colony Morphology: Colonies are typically flat, with a spreading edge and a metallic sheen.
• Pigment Production: The production of pyocyanin (a blue-green pigment) and fluorescein (a yellow-green pigment) is enhanced, giving colonies a distinctive coloration under UV light.

Applications

Cetrimide agar is widely used in:

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

Limitations

While cetrimide agar is effective for isolating Pseudomonas aeruginosa, it is not suitable for differentiating between different strains of Pseudomonas. Additional biochemical tests and molecular methods are required for further characterization.

Related Pages

• Pseudomonas aeruginosa
• Microbiological culture
• Selective medium