Glucose Fermentation Broth (like Glucose O/F Medium) includes the following major ingredients:
- Glucose is a sugar from which most common chemoheterotrophic bacteria can obtain energy by fermentation and/or respiration. Glucose can also be utilized as a source of carbon, but these media include a large number of potential carbon sources (amino acids as well as glucose), and whether or not glucose is used as a carbon source cannot be directly determined from the reactions seen in these media.
- Peptone is a commonly-used medium ingredient which mainly supplies amino acids (sources of nitrogen, carbon, sulfur and energy for many bacteria). A minimum amount of peptone should be incorporated in the medium, as overabundance of ammonium (alkaline) released from deamination of amino acids can neutralize the acids.
- pH indicator: Brom-thymol blue turns yellow with a net acidic pH and blue with a net alkaline pH.
Testing whether an organism can ferment glucose is one of the basic, primary tests in the identification of chemoheterotrophic bacteria. For this test we routinely use a "Glucose Fermentation Broth."
- Fermentation of glucose results in the abundant production of acidic end products, the presence of which can be detected by the pH indicator in the medium.
- Many organisms produce gas – either CO2 alone or a mixture of H2 and CO2. H2 is insoluble and is detected by bubble formation in a Durham tube placed in the medium.
Note the examples shown below.
- Tube 1: No fermentation. The pH indicator remains purple. There can still be growth due to the use of amino acids as sources of energy (usually by respiration).
- Tubes 2A and 2B: Fermentation with the production of acid (yellow color) but no gas. A slight amount of acid is seen in tube 2A, but fermentation is still recorded for this tube.
- Tubes 3A and 3B: Fermentation with the production of acid (yellow color) and insoluble gas (bubble in Durham tube). Tube 3B shows an alkaline reaction on top; this is simply due to deamination of amino acids whose alkaline reaction has not been over-neutralized by the acid diffusing through the tube from fermentation.