Final answer:
When MacConkey agar turns pink or purple, it indicates a decrease in pH due to lactose fermentation by certain bacteria. The pH indicator in the agar responds to acidic products, resulting in a color change.
Step-by-step explanation:
When the medium turns pink or purple in MacConkey agar, the pH generally goes down. This color change indicates that lactose fermentation has occurred, producing acid as a metabolic byproduct, which lowers the pH of the medium. MacConkey agar contains a pH indicator that responds to changes in the medium's acidity. Bacteria that ferment lactose, such as Escherichia coli, produce acidic compounds, leading to the pink or purple hue. This is distinct from the Gram staining process during which Gram-positive cells appear purple due to retention of the crystal violet-iodine complex, while Gram-negative cells appear pink because they are decolorized and counterstained with safranin.
The pKin (pK) of the indicator within the MacConkey agar determines at which pH the color change occurs. Indicators, such as phenolphthalein, change color at specific pH levels. This occurs when the concentration of hydronium ions in the medium reaches a level that the indicator is sensitive to, signifying a decrease in pH.