Cetrimide agar is used to determine the ability of an organism to grow in the presence of cetrimide, a toxic substance that inhibits the growth of many bacteria by causing the release of nitrogen and phosphorous, which slows or kills the organisms because organisms other thanP. aeruginosaare unable to withstand this germicidal activity, while P. aeruginosa is resistant to cetrimide. An incubator full of tall stacks of agar plates takes longer to warm up than an incubator with small stacks, and the plates in the middle of the stacks will also take longer to warm up. By using a standardized inoculum of 10-100 CFU, you can avoid the unpleasant surprise of finding 120 colonies on your agar plate the day after you inoculated it with the suspension prepared with a turbidimeter. When cetrimide is in contact with bacteria, nitrogen and phosphorus are released from the bacterial cell. rev2023.3.3.43278. Why or why not? Non-Lactose fermenting bacteria such as Salmonella, Proteus species and Shigella cannot utilize lactose, and will use peptone instead. No strict requirement was deliberately given in this chapter because the test is qualitative, not quantitative. XLD agar is composed of yeast extract, sodium chloride, xylose, lactose, sucrose, l-lysine hydrochloride, sodium thiosulfate, iron (III) ammonium citrate, phenol red, sodium deoxycholate, agar, and distilled or deionized water. 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. Media: Eosin, Methylene Blue, lactose, sucrose, Reagents/Indicators: Eosin Y and Methylene Blue. Indicators form a dark purple precipitate at low pH (due to fermentation products) and also inhibit gram positive bacteria. 41 a leg culture from a nursing home patient grew 293 0 obj <>stream Why is MSA optional only during the unknowns? Welcome to Biology.SE! Mary E. Allen. As the R&D Scientist, she works on both new products and product and process improvements. Some people supplement it with extra sucrose or sodium chloride, but I think these are mere customs rather than experimentally-proven improvement.

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