About this test
What is the purpose of the test?
The purpose is to see if the microbe can ferment the carbohydrate (sugar) lactose as a carbon source.
How is lactose fermentation determined?
If lactose is fermented to produce acid end products, the pH of the medium will drop. A pH indicator in the medium changes color to indicate acid production.
What medium is used?
Several media are available for this. Most commonly used is phenol red lactose broth. The medium is a nutrient broth to which 0.5-1.0% lactose 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.
How is the test performed?
An inoculum from a pure culture is transferred aseptically to a sterile tube of phenol red lactose broth. The inoculated tube is incubated at 35-37 C for 24 hours and the results are determined. A positive test consists of a color change from red to yellow, indicating a pH change to acidic.
What reagents are added?
Performing this test in the VUMIE Online lab
Inoculation of Medium
1. Select the phenol red lactose 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. No reagents are required for completion of this test.
8. Positive tests will be yellow; negative results are any other color.