Fig. 18: Proteus vulgaris
Growing on MacConkey Agar
MacConkey agar is a selective medium used for the isolation of non-fastidious Gram-negative rods, particularly members of the family Enterobacteriaceae and the genus Pseudomonas, and the differentiation of lactose fermenting from lactose non-fermenting Gram-negative bacilli. MacConkey agar contains the dye crystal violet well as bile salts that inhibit the growth of most Gram-positive bacteria but do not affect the growth of most Gram-negatives.
If the bacteria do not ferment lactose, the colonies and confluent growth appear colorless and the agar surrounding the bacteria remains relatively transparent (arrow).