What do lactose fermenting bacteria do to the pH of MacConkey Agar?

Lactose fermenting bacteria lower the pH of MacConkey Agar as a result of their metabolic activities. During lactose fermentation, these bacteria break down lactose into lactic acid and other acids, acidifying the environment. This acid production leads to a drop in pH. In MacConkey Agar, the pH indicator is neutral red, which turns red at lower pH levels (below 6.8) indicating that acid is present. Therefore, the presence of lactose fermenting bacteria is indicated by color changes in the medium, specifically leading to a red or pink coloration surrounding the colonies. This is a key aspect of identifying lactose fermenters, as it distinguishes them from non-fermenters which do not produce acid and will not change the pH significantly.