Agar Medium N (Cetrimide Agar)500 G

Cetrimide Agar was described by King et al (1). This media is based on the formulation described in BP(3) and is in accordance with the harmonized method of USP/EP/IP/JP (2,4,5,7). It is used as a selective medium for the isolation of Pseudomonas aeruginosa from pharmaceutical products. This medium is also used for microbial limit testing for non-sterile products.Lowburry first reported the use of cetrimide as an agent for selective isolation of Pseudomonas (6). This medium is also used for determining the ability of an organism to produce fluorescein and pyocyanin. Cetrimide (N-acetyl-N, N,Ntrimethylammonium bromide) is incorporated in the medium to inhibit bacteria other than Pseudomonas aeruginosa. This compounda cationic detergent acts as a quaternary ammonium compound, which causes nitrogen and phosphorus to be released from bacterial cells other than Pseudomonas aeruginosa. Magnesium chloride and potassium sulphate incorporated in the medium enhances the production of pigment pyocyanin, which is a blue-green pigment, diffusing into the medium. This improves detection of Pseudomonas on this medium. Presence of magnesium ions can also neutralize EDTA, if present in the sample. Pancreatic digest of gelatin provides the essential nutrients for growth of Pseudomonas, while glycerin serves as slow and continuous carbon source for the growing cell.For the isolation of Pseudomonas aeruginosa, plates of Cetrimide Agar should be inoculated from non-selective medium such as Soyabean Casein Digest Medium (M011B). If the count is high the test sample can be directly inoculated onto this medium. Pseudomonas aeruginosa colonies may appear pigmented greenish (under UV light also). Addition of nalidixic acid can aid in inhibiting the growth of accompanying flora.Storage and Shelf-life: Store below 30°C in tightly closed container and the prepared medium at 2-8°C. Use before expiry date on the label.