MacConkey Agar

It is used to selectively isolate Gram-negative and enteric bacilli.

It is used as a selective solid medium for the growth and enumeration of coliform group bacteria and especially Escherichia coli in microbiological analysis.

Among the media prepared for coliform bacteria, it has the lowest selectivity.

It is also used for isolation of Salmonella and Shigella as well as coliform bacteria from feces, urine, waste water and food samples.

At the end of 18-24 hours incubation at 35 C, lactose-negative bacteria form colorless colonies, while lactose-positive colonies become red and surrounded by a turbid zone due to the precipitation of bile acids due to the decrease in pH.