On a TSI slant, if lactose or sucrose are fermented, what change might be observed?

• A. Slant may turn yellow as well.
• B. Slant remains green.
• C. Slant turns red.
• D. No color change in slant.

Answer: (A)

Fermentation of lactose or sucrose produces acid, which lowers the pH and turns the phenol red indicator yellow. Since lactose and sucrose are present in the slant portion of the tube, fermentation of these sugars can cause the slant itself to turn yellow as well. If only glucose is fermented, acid forms mainly in the butt and the slant can stay red because aerobically met metabolism in the slant shifts toward alkaline products.