MacConkey Broth is a differential medium containing neutral red for the detection of coliform organisms in water and the examination of milk. It is a selective culture medium used as a presumptive test for E. coli and coliform bacteria and to determine the E. coli or coliform titer of milk, water and other materials. This broth contains lactose which, when degraded, produces acid and gas, as defined to indicate the presence of E. coli. The gas formed is collected in DURHAM tubes and acid production is detected by the purple bromocresol indicator, which turns yellow. Beef bile promotes the growth of several species of intestinal bacteria and inhibits the growth of microorganisms, which do not inhabit the intestine. A change in color from purple to yellow indicates the growth of a bile-tolerant, lactose-fermenting organism such as Escherichia coli. Non-lactose-fermenting organisms, such as Salmonella and Shigella, do not affect the appearance of the environment.