The methyl red test is used primarily to distinguish various coliforms. They all ferment glucose and the pH of the medium drops quickly.when methyl red is added after overnight incubation the cultures of all these organisms will be posit ive. Escherichia coli will continue to ferment glucose and enough acid may be formed to sterilize the medium. Klebsiella aerogens cultures decaroxylate and condense the pyruvic acid to form acetylmethylcarbinol which causes the pH to rise. when met hyl red is added the yellow colour indicates a negative reaction. see also Voges Proskaur