Carbohydrates are the prime sources of energy and carbon skeletons for the synthesis of cellular substances. In this exercise, the reactions of several microorganisms with disaccharide(lactose) and monosaccharide(glucose) will be studied. Detecting acid formation from these sugars can be done easily by including a pH indicator in the test medium. A pH indicator commonly used is phenol red, which is yellow at pH 6.9(acid) and red at pH 8.5(alkaline). Gas production in a carbohydrate medium may be detected through the use of an inverted(Durham tube that serves to trap any gas formed.