MacConkey Broth is a liquid medium recommended for the cultivation of Gram-negative, lactose-fermenting enteric bacteria in samples of water, milk, and foods. Enzymatic Digest of Gelatin supplies nitrogen and vitamin sources in this medium. Lactose is the main carbohydrate source. Its fermentation produces organic acids, which lower the pH of the medium and shift the color of the indicator Bromocresol Purple from purple to yellow. Oxbile inhibits the growth of Gram-positive organisms.