On MacConkey agar, the growth of gram-negative bacteria that digest lactose is shown by pink colonies.The agar is made up of bile salts, lactose, and neutral red, a pH indicator.This is the justification:
MacConkey agar is a culture medium that is both selective and differential.In addition, it distinguishes between different bacteria according to their capacity to ferment the sugar lactose.Lactose can be broken down by lactose fermenters, which will produce lactic acid and other acidic byproducts.While most gram-positive bacteria are inhibited, gram-negative bacteria are allowed to proliferate.As a result, the surrounding medium's pH drops.