Final answer:
The KIA medium turns yellow when there is glucose utilization with acid production due to the fermentation by bacteria, leading to a drop in pH below 6.8.
Step-by-step explanation:
When KIA medium is used to test for glucose utilization with acid production by bacteria, the color change observed can indicate various biochemical activities. Specifically for glucose utilization with associated acid production, the medium will exhibit a color change to yellow. This is because the pH indicator within the KIA medium turns yellow at pH measurements below 6.8 when acids are produced as a byproduct of fermentation by the bacteria.
This yellow color change is denoted by the letter A (acidic reaction) in the recording of the test outcome which is reported as the appearance of the slant/appearance of the butt.