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On Macconkey agar, lactose fermenting bacteria turn the agar bright red. Why does non-lactose fermenting bacteria turn the MaCconkey agar yellow?. Does it have to do with the alkalinity of waste products?

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    $\begingroup$ I'm voting to close this question as off-topic because it belongs on Stack Exchange Biology. There is nothing wrong with the question, just wrong site. $\endgroup$
    – M__
    Mar 19, 2020 at 22:10

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McConkey agar is already red. It's trick is that it has two carbon sources. The bacteria that cannot use the more energetically useful lactose use the amino acids (peptone) generating ammonia which changes the pH and the crystal violet goes yellow. The ones that can use lactose acidify the agar a bit so go a bit more red —but not much.

This being the SE for bioinformatics, it should be said that if the genome of the organism in question is sequenced and deposited in NCBI you can find if the organism possesses the enzyme that allows the hydrolysis of lactose (beta-galactosidase) by doing a protein BLAST. The lacZ gene encoded protein in E. coli is NP_414878 in NCBI. This goes in the accession number input box. The species in question goes in the organism box. If you get something with good coverage (80%) with more than 20% homology the organism is likely lac+.

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