About this test
What is the purpose of the test?
The purpose is to see if the microbe can ferment the carbohydrate (sugar) glucose (also known as dextrose) as a carbon source, and to see if the fermentation products include gases.
How is gas production from glucose determined?
When many microbes ferment glucose ( dextrose ) , they produce gases, mainly carbon dioxide and hydrogen. These gases bubble up through the medium and escape into the atmosphere. Tubes of broth media can be made with inverted tubes called Durham tubes which are filled with medium after sterilization. Gas-producing microbes generate enough gas to force the medium from the tube, filling it with bubbles.
What medium is used?
Several media are available for this. Most commonly used is phenol red glucose broth. The medium is a nutrient broth to which 0.5-1.0% glucose is added. The pH indicator phenol red is red at neutral pH but turns yellow at pH <6.8. It also changes to magenta or hot pink at pH >8.4. Additionally, this medium contains a Durham tube , a smaller inverted tube which can serve as a trap for gas bubbles generated during fermentation of sugars.
How is the test performed?
An inoculum from a pure culture is transferred aseptically to a sterile tube of phenol red glucose (dextrose) broth. The inoculated tube is incubated at 35-37 C for 24 hours and the results are determined. A positive test consists of a yellow, acidic medium with visible displacement of the gas from the Durham tube.
What reagents are added?
Performing this test in the VUMIE Online lab
Inoculation of Medium
1. Select the phenol red glucose broth medium.
2. Complete the process of a tube-to-tube aseptic transfer to inoculate the medium.
Incubation of the Inoculated Medium
3. Place the inoculated tube into the 35-37 C incubator.
4. Press the New Day button to move forward 24 hours.
Determination of Test Results
5. Incubate this test for only 24 hours.
6. Retrieve the incubated culture from the incubator.
7. Observe the color of the culture and the condition of the Durham tube. A positive test is indicated when the tube is yellow and bubbles are present in the Durham tube. Any other result is negative for the generation of gas from glucose.