What color change is a negative result in the KIA test for lactose fermentation?

In the Kligler's Iron Agar (KIA) test, lactose fermentation is indicated by a color change in the medium. A negative result for lactose fermentation will typically manifest as a color change to purple. This occurs because the pH of the medium remains neutral, indicating that lactose has not been fermented by the test organism.

When lactose is fermented, acid is produced, which lowers the pH and causes the phenol red indicator in the medium to turn yellow. Therefore, if lactose is not fermented, the medium does not change to yellow, and the alkaline condition remains, which retains the purple color of the medium. This distinct observation is crucial for characterizing the metabolic capabilities of bacteria during the microbiological analysis.