Cetrimide Agar (also called Pseudosel Agar or Pseudomonas Cetrimide Agar) is a selective and differential medium for the isolation and identification of Pseudomonas aeruginosa, a gram-negative bacterium, in a variety of products. However, Serratia, Klebsiella and Acinetobacter can be cultured. As its name suggests, this medium contains cetrimide (Cetyltrimethylammonium bromide), a quaternary ammonium-based surfactant, which acts as a selective agent against alternative microbial flora due to its detergent properties. Cetrimide also increases the production of Pseudomonas pigments such as pyocyanine and pyoverdine, which have a characteristic blue-green and yellow-green color. Pseudomonas aeruginosa produces a number of water-soluble iron chelating agents, including yellow-green or yellow-brown fluorescent pyoverdine. When pyoverdine combines with water-soluble blue pyocyanine, the characteristic bright green color of Pseudomonas aeruginosa is created.  Cetrimide agar is widely used in the analysis of cosmetics, pharmaceuticals and clinical samples for testing the presence of Pseudomonas aeruginosa.