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. Inoculate each of the above bacteria into separate tubes of glucose, lactose and sucrose broth. Examine the tubes for acid and gas production. Go back to the microbiology home page .