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Bromothymol blue was added in order to reduce false positives. The citrate agar is green before inoculation, and turns blue, because of BTB as a positive test indicator, meaning citrate is utilized. The test is also prepared on a slant to maximize bacterial growth for an even better indication of the use of citrate.
Bromothymol blue (also known as bromothymol sulfone phthalein and BTB) is a pH indicator. It is mostly used in applications that require measuring substances that would have a relatively neutral pH (near 7). A common use is for measuring the presence of carbonic acid in a liquid.
Bacteria are inoculated on a medium containing sodium citrate and a pH indicator such as bromothymol blue. The medium also contains inorganic ammonium salts, which are utilized as sole source of nitrogen. Use of citrate involves the enzyme citrate lyase, which breaks down citrate to oxaloacetate and acetate.
H + + Bromothymol blue (Blue) → Bromothymol blue-H + (Yellow) Elevated protein concentrations produce slightly elevated specific density results as a consequence of the indicator's protein error; in addition, samples with a pH above 6.5 give lower readings as a result of the indicator's bias.
Bromothymol blue is the indicator used in the agar, it changes to yellow in case of acid production during fermentation of lactose or changes to deep blue in case of alkalinization. Lactose-positive bacteria build yellow colonies. Bacteria which decarboxylate L-cystine cause an alkaline reaction and build deep blue colonies. [1]
Use of citrate results in the creation of carbonates and bicarbonates as byproducts. Organisms degrading citrate must also use the ammonium salts, producing ammonia, [10] thus increasing the pH of the medium. [11] The increase in pH then causes color change in the bromothymol blue indicator, turning it blue.
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Rumours of the origin of urine indicator-dye go back at least as far as 1958, [1] and the story is commonly told to children by parents who do not want them to urinate in the pool. [3] A 1985 biography of Orson Welles describes him using such a dye as part of a prank in 1937. [1] [6]