Cetrimide Medium (without Membrane Filter)

Principle And Interpretation

Field of Application: Water, pharmaceuticals, cosmetics and other samples. DriFilter Membrane Nutrient Pad Medium are ready to use sterile culture media in the form of a 50 mm biological inert absorbent pads impregnated with standard culture medium, then dried and sterilized in 55 mm petri plate. They eliminate the need of laborious media preparation and autoclaving procedures. The nutrient pads are to be just rewetted with sterile distilled water and are ready to use. Use of nutrient pads allows larger sample volumes to be tested at a time. Interpretation of results is directly by counting the CFUs and also quantifies the microbial load present in the sample. Pseudomonas aeruginosa grows well on all normal laboratory media but specific isolation of the organism, from environmental sites or from human, animal or plant sources, is best carried out on a medium, which contains a selective agent and also constituents to enhance pigment production. Most selective media depend upon the intrinsic resistance of the species to various antibacterial agents. Cetrimide inhibits the growth of many microorganisms whilst allowing Pseudomonas aeruginosa to develop typical colonies. Cetrimide 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. Cetrimide Agar developed by Lowburry (1) is a modification of Tech Agar (Medium A) with addition of 0.1% cetrimide for selective isolation of P. aeruginosa. Later, due to the availability of the highly purified cetrimide, its concentration in the medium was decreased (2). The incubation was carried out at 37°C for a period of 18-24 hours (3).