Microbiology - 007 - Carbohydrate Fermentation Test

The carbohydrate fermentation test is used to determine whether or not a bacteria can utilize a certain carbohydrate. It tests for the presence of acid or gas produced from carbohydrate fermentation. The media in each tube contains a single carbohydrate – in this case glucose, lactose, and sucrose. An indicator, phenol red, is also present that will detect a change in pH due to acid production, and small, inverted tubes called Durham tubes test for the production of gas. Use a sterile loop to inoculate each tube with a bacterial culture. Incubate the tubes at 37 degrees Celsius for 1-2 days. After incubation, if the liquid in the tube has turned yellow, the pH in the tube has dropped, changing the color of the phenol red indicator. This indicates that the bacteria can ferment the carbohydrate in the tube, producing acid. Otherwise, the tube will remain red if the bacteria cannot ferment the carbohydrate, a negative result. If the bacteria also produces as, it will collect in the Durham tube. A bubble in the Durham tube, no matter how small, is a positive result for gas production. If there is no bubble, the bacteria does not produce gas from fermentation of this carbohydrate. In this example, the bacteria ferments glucose and sucrose, but not lactose. It also produces gas when fermenting sucrose, but not glucose.