Cetrimide Agar is used for selective isolation of Pseudomonas aeruginosa from clinical samples. The medium is a modification of the medium proposed by Brown and Lowbury (1965) for the isolation and differentiation of Pseudomonas aeruginosa from various materials. Cetrimide Agar contains quaternary ammonium, cationic detergent compound like cetrimide which has a property to inhibit all the bacteria except P. aeruginosa thus making the medium selective for Pseudomonas aeruginosa. Cetrimide causes release of nitrogen and phosphorus from bacterial cells other than P. aeruginosa. Cetrimide Agar is recommended in the United States Pharmacopoeia XXVI and European Pharmacopoeia IV for use in Microbial Limit Tests. The formulation is also in the A.O.A.C. guidelines for isolation of Pseudomonas
Pancreatic digest of gelatin is a source of carbon and nitrogen for the growth of microorganisms. Agar is a solidifying agent. Sodium chloride maintains osmotic equilibrium in the medium. Magnesium chloride and Potassium sulphate stimulates the bacterium to produce and secrete a pigment called pyocyanin due to which the colony colour turns light green, and also they forms a grape like odour. Cetrimide Agar is a quaternary ammonium salt, which acts as a cationic detergent that reduces surface tension in the point of contact and has precipitant, complexing and denaturing effects on bacterial membrane proteins. It exhibits inhibitory actions on a wide variety of microorganisms including Pseudomonas species other than Pseudomonas aeruginosa.