|The identification of some bacteria is aided by determining what nutrients the bacteria can utilize and what end products will be produced in the process. These characteristics are controlled by the enzymes which the bacteria produce. Because the type of enzyme(s) bacteria produce is genetically controlled, the pattern of sugars fermented may be unique to a particular species or strain. Fermentation products are usually acid (lactic acid, acetic acid etc.), neutral (ethyl alcohol etc.), or gases (carbon dioxide, hyrogen, etc.).
To determine the products of sugar fermentation, a carbohydrate fermentation broth is prepared at pH 7.4. This broth contains 3 essential ingredients: 0.5%-1.0% of the carbohyrate to be tested (e.g. lactose or glucose), nutrient broth, and the pH indicator phenol red. The nutrient broth, which is a light red color, supports the growth of most organisms whether they are able to ferment the sugar or not.
The test organism is inoculated into a broth containing the test sugar and incubated. A bright yellow color indicates the production of enough acid products from fermentation of the sugar to drop the pH to 6.9 or less. Production of gas is determined with a Durham tube , a small inverted vial filled with the carbohydrate fermentation broth. If gas is produced during fermentation of the sugar, it is trapped at the top of the Durham tube and appears as a bubble. Slow fermenters may take a week or more to cause color changes detectable by the human eye. Positive (yellow color or yellow color with gas bubble) and negative results (red color, no gas bubble) are shown in this image.