Module 9, Experiment 8:
Carbohydrate Fermentation Testing
Preparation of Bacterial Cultures:
Incubate the culture tubes at 35°C - 37°C for 12 hours. Do not let the cultures incubate for more than 24 hours as you may not get an accurate result!
Record your observations here.
A. What is fermentation? It is a metabolic process that allows the production of ATP without need for oxygen.
B. What is the reason that it is important to not incubate the fermentation tubes beyond 24 hours?
You may not get accurate results. Although sugar is the primary nutrient used, when a microbe runs out of it, protein or other nutrients will be attacked. This causes changes in the color of the medium because of the pH indicator added to detect acid production. When proteins are used, alkaline by-products are produced and the medium can change colors. If you let these sugar tests go for more than a day, you risk the possibility that the color will have changed and you may misread the test result.
C. What is the purpose of adding phenol red to the fermentation tubes?
Phenol red is added as a pH indicator. It will turn yellow below pH 6.8 and a darker-pinkish red above pH 7.4.
D. What is the reason that bacteria have differences in the carbohydrates that they can ferment?
The ability or inability of a particular species to ferment a particular carbohydrate depends on the presence of the enzymes needed for a particular fermentation pathway. As it is the DNA of an organism that codes for which particular enzymes that organism contains, the presence or lack of a particular enzyme is decided at the genetic level and varies by species.
E. What is the reason that the formation of yellow color indicates fermentation?
Acid production produces a color change from red to yellow, indicating the organism is capable of metabolizing the sugar in the tube.
F. What information can be gained by