Question

If a bacteria is positive for mannitol fermentation, when it is incubated in CTA mannitol, the media turns yellow. This is because:

		When the bacteria comes into contact with mannitol, it produces a yellow-colored pigment.
		Fermentation of mannitol produces yellow metabolites.
		Fermentation of mannitol produces acids, which lower the pH of media. Therefore, the pH indicator turns yellow.
		Mannitol turns the media yellow.

Answer 1

If a bacteria is positive for mannitol fermentation , the media turns yellow when the bacteria is incubated in CTA mannitol because of the production of acids from mannitol , which lowers the pH of media therefore the pH indicator turns yellow.

In the CTA Mannitol phenol red is mixed as pH indicator which has red color on pH range 7 to 8.2(Basic). When fermentation occurs the acids are formed which decrease the pH of medium and make it less than 7.So the phenol red now turns yellow in the acidic pH.