Page 119 - MNUmicrobiology practical 2025

General Microbiology & Immunology (PM 401) second level Semester 4 2024/2025

B- Methyl Red Test
• Purpose: To determine mixed acid fermentation (lactic, acetic, formic, etc).

• Media: MRVP broth--buffered peptone glucose broth used for both MR and VP

tests.

• Reagents/indicators: Methyl Red- red in pH under 4.4, yellow in pH over 6.2, and

orange in between

• Mechanism/reactions: If the organism uses the mixed

acid fermentation pathway and produces large

amounts of organic acids from glucose, the acids will

overcome buffers in the medium and the culture will

be acidic.

• Interpretation:

(+) = bright red color immediately upon the addition of
methyl red (pH < 4.4)

(-) = yellow color (pH > 6.2)
Weak (+) = orange color

Note

• E. coli is one of the bacteria that produces acids, causing the pH to drop below 4.4. When the

pH indicator methyl red is added to this acidic broth it will be cherry red (a positive MR test).

• Klebsiella and Enterobacter produce neutral products from glucose (e.g. ethyl alcohol, acetyl
methyl carbinol). causing the pH to rise above 6.2. At this pH, methyl red indicator is a yellow

color (a negative MR test).

C- Voges Proskauer Test

• Purpose: To detect the production of acetoin (acetylmethyl carbinol) or 2,3

butainediol (acetoin is the precursor) from glucose broth.

• Media: MRVP broth

General Microbiology & Immunology (PM 401) Practical note Page 108