<P> Using neutral red pH indicator, the agar distinguishes those Gram - negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac -) fermentation . </P> <P> This medium is also known as an "indicator medium" and a "low selective medium". Presence of bile salts inhibits swarming by Proteus species . </P> <P> By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of pink colonies . The bile salts precipitate in the immediate neighbourhood of the colony, causing the medium surrounding the colony to become hazy . </P> <P> Non-lactose fermenting bacteria such as Salmonella, Proteus species, Yersinia, Pseudomonas aeruginosa and Shigella . </P>

Gram positive bacteria that can grow on macconkey agar