Some bacteria perform MIXED-ACID FERMENTATION. The by-products are mixtures of large amounts of stable acids. Other fermentative organisms produce smaller amounts of less stable acids.
The Methyl-Red test tests for the ability to perform mixed-acid fermentation. MR-VP broth contains glucose, peptone, and a phosphate buffer. Organisms that perform mixed-acid fermentation produce enough acid to overcome the buffering capacity of the broth, so a decrease in pH results. Organisms that perform other kinds of fermentation cannot overcome the buffering capacity of the broth.
After incubation, the pH indicator Methyl Red is added to the broth. Methyl Red is red at pH below 4.4 (this would be a positive result) and yellow at pH above 6.0. An orange color indicates an intermediate pH and would be considered a negative result.
This test is among a suite of tests (Indole, Methyl-Red, Vogues-Proskauer, and Citrate) that are used to differentiate among the Gram-Negative bacilli in the family Enterobacteriaceae.
+ = Positive (Red)
- = Negative (Orange to Yellow)
V = Variable
1. Obtain two MR-VP broths from the back shelf.
2. Inoculate one broth using aseptic technique. Leave the other broth uninoculated (this will be a control).
3. Incubate at appropriate temperature (whatever temperature your organism grows well at). Incubate for two to five days.
4. Obtain your broths from the incubator.
5. Add a dropperful of Methyl Red to each broth. Methyl Red is found in small dropper bottles on the back shelf.
6. Observe the color (which should develop within a few minutes).