What happens to the pH of KIA medium when only glucose is fermented?

When only glucose is fermented in Kligler's Iron Agar (KIA) medium, the upper slant typically remains neutral or alkaline while the butt of the medium becomes acidic. This occurs because glucose fermentation produces acids that primarily drop the pH in the anaerobic butt region, causing it to turn yellow. In contrast, the aerobic slant remains red or neutral due to the oxidation of any acids produced earlier, leading to an alkaline reaction. Hence, the upper slant does not become acidic since there's insufficient fermentation of lactose (which gets utilized after glucose), allowing it to stay neutral/alkaline. This demonstrates how KIA medium differentiates between carbohydrate fermenters and can provide insight into the metabolic capabilities of bacteria being tested.