However, many aerobic bacteria are capable of fermentation, and may do so even when oxygen is available. In this experiment, we will examine the capability of bacteria to ferment a variety of carbohydrates. We will also determine the end products of bacterial fermentation. Most bacteria produce organic acids as by-products of fermentation. Incorporation of a pH indicator into a medium allows detection of acidic fermentation products.
Phenol red is one of the most commonly used indicators.
At acid pH, phenol red changes from red to yellow. Other indicators may be used as well. Many bacteria also produce gases in addition to acids. Gases may be detected by placing an inverted glass tube, called a Durham tube, into broth tubes. Acid and gas production from different carbohydrates are important bacterial identification characteristics.