Final answer:
Acid production by carbohydrate fermentation in a KIA slant turns the medium yellow, indicating an acidic reaction. This color change occurs due to a pH shift caused by acid production, when sugars are fermented by the bacteria.
Step-by-step explanation:
The production of acid by carbohydrate fermentation in a KIA (Kligler's Iron Agar) slant will turn the medium yellow. KIA slants are used to differentiate bacteria based on their ability to reduce sulfur and ferment carbohydrates.
When the acid is produced from fermentation, the pH indicator in the medium will show a color change. A yellow color in the slant and/or butt indicates an acidic reaction due to the fermentation of the carbohydrates, leading to a drop in pH.
Moreover, if the bacteria can only ferment glucose (and not the other sugars in the medium), typically the yellow color will only be present in the butt of the tube, because glucose is present in the lowest concentration and will be exhausted quickly, leading to an acidic condition only in the anaerobic environment (the butt).
If the organism ferments lactose or sucrose, which are present in higher concentrations, the entire slant as well as the butt will turn yellow. H2S production, if present, will be evident as blackening of the medium, and gas production will result in cracks or displacement of the agar.