The methyl-red(MR) and Voges-Proskauer(VP) tests are used to differentiate two major types of facultatively anaerobic enteric bacterial species based on their pattern of glucose metabolism.
Methyl Red Test
All enterics initially produce pyruvic acid from glucose metabolism.
Glucose -----Glucose Metabolism----> Pyruvic acid
Positive result: Some enterics subsequently use the mixed acid pathway to metabolize pyruvic acid to other acids(lactic, acetic, and formic acids).
Pyruvic acid -----Mixed Acid Pathways------> lactic, acetic, and formic acids
Many acids (pH 4.2) + added methyl red = red color
Bacteria are called methyl-red-positive.
Negative result: Other enterics subsequently use butylene glycol pathway to metabolize pyruvic acid to neutral end-products.
Pyruvic acid------Butylene Glycol Pathway------> neutral end-products
Neutral end-products (pH 6.0) + added methyl red = yellow color
Bacteria are called methyl-red-negative.
Methyl Red Test
All enterics initially produce pyruvic acid from glucose metabolism.
Glucose -----Glucose Metabolism----> Pyruvic acid
Positive result: Some enterics subsequently use the mixed acid pathway to metabolize pyruvic acid to other acids(lactic, acetic, and formic acids).
Pyruvic acid -----Mixed Acid Pathways------> lactic, acetic, and formic acids
Many acids (pH 4.2) + added methyl red = red color
Bacteria are called methyl-red-positive.
Negative result: Other enterics subsequently use butylene glycol pathway to metabolize pyruvic acid to neutral end-products.
Pyruvic acid------Butylene Glycol Pathway------> neutral end-products
Neutral end-products (pH 6.0) + added methyl red = yellow color
Bacteria are called methyl-red-negative.
http://web.clark.edu/tkibota/images/MethylRed.gif
Voges-Proskauer Test
A test to distinguish between acetoin-producing enterics and enterics that do not produce acetoin. (Acetoin is otherwise known as acetylmethylcarbinol) Enterics that subsequently produce neutral end-products from pyruvic acid are detected with the Voges-Proskauer test.
Positive result:
Glucose ------Glucose Metabolism-------> Pyruvic Acid
Pyruvic acid ---------------> Acetoin
Acetoin + added alpha-naphthol + added KOH = red color
Negative result:
Glucose ------Glucose Metabolism-------> Pyruvic Acid
Pyruvic acid -----------------> No Acetoin
No acetoin + added alpha-naphthol + added KOH = copper color
Voges-Proskauer Test
A test to distinguish between acetoin-producing enterics and enterics that do not produce acetoin. (Acetoin is otherwise known as acetylmethylcarbinol) Enterics that subsequently produce neutral end-products from pyruvic acid are detected with the Voges-Proskauer test.
Positive result:
Glucose ------Glucose Metabolism-------> Pyruvic Acid
Pyruvic acid ---------------> Acetoin
Acetoin + added alpha-naphthol + added KOH = red color
Negative result:
Glucose ------Glucose Metabolism-------> Pyruvic Acid
Pyruvic acid -----------------> No Acetoin
No acetoin + added alpha-naphthol + added KOH = copper color
