Lactose-fermenting bacteria turn eosin-methylene blue medium from colorless to b
ID: 167670 • Letter: L
Question
Lactose-fermenting bacteria turn eosin-methylene blue medium from colorless to black or red. What causes this change?
C. Lactose fermentation generates alcohol. The alcohol reacts with chemicals in the medium to cause the color change.
A. Lactose fermentation generates lactate, making the medium more alkaline and causing the color change. B. Lactose fermentation generates lactic acid. Lactic acid lowers the pH of the medium, causing the medium to change colors.C. Lactose fermentation generates alcohol. The alcohol reacts with chemicals in the medium to cause the color change.
D. Lactose fermentation allows pyruvate to build up. This excess of pyruvate causes the color change.Explanation / Answer
Eosine methylene blue medium is a selective agar medium which contains the dye eosin and methylene blue. It inhibits the growth of gram positive bacteria because of the presence of aniline dye but is selective for gram negative species. In this case, rapid lactose fermentation results in the production of acids which lowers the pH thereby encouraging dye absorption by the colonies which results into black or red color.
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