Without carbohydrate in the broth (some) bacteria would then degrade the peptone, which from its end products would increase pH and then turn zhe phenol red pink. So I think this is some type of control just to make sure the test parameters can produce a pink broth. Is there something more here that I'm missing though? The question is asking what function phenol red broth without carbs serves, and that's only thing I can think of. Is this correct or is there something else that is more relevant?

  • $\begingroup$ Indicators in media are usually used to monitor the degradation of the media due to growth. If the pH falls too much, living conditions are much worse to the point that the cells will die. So this is more a quick check. $\endgroup$ – Chris Dec 22 '16 at 19:16

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