Carbohydrate Fermentation Testing

A. What is fermentation?
It is a metabolic process that allows the production of ATP without need for oxygen

B. Why is it important not to 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 changes 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. Why is phenol red added 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. Why do bacteria have differences in the carbohydrates they can ferment? The ability of 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 in 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. Why does the formation of yellow color indicate 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 running a fermentation series on a particular microbe? For each carbohydrate tested, the following information can be learned: -can the organism ferment the particular carbohydrate

-if so, does it produce acidic end or by-products
-if so, does it produce gaseous products

G. What does a dark pink or red color indicate?
A pH above 7.4. This darker color indicates an alkaline or basic metabolic product which is due to the utilization of the peptone, rather than the sugar

H. What is the source of the air bubble that may form in the Durham tube? A bubble that indicates gas production

I. Based on your results, what is the carbohydrate profile for S. epidermidis? Glucose AG, Fructose A, and mannitol

J. Based on your results, what is the carbohydrate profile for S. cerevisiae Glucose AG, fructose A, and mannitol