exercise 30 KTTK: Methyl Red and Voges-Proskauer tests (micro. lab)

Methyl Red and Voges-Proskauer (MR-VP) broth is

combination medium

what the MR-VP broth is used for

used for both Methyl Red and Voges-Proskauer tests

 what the methyl red (MR) test is designed to detect and how it is set up

organisms capable of mixed acid fermentation producing acids and products such as

• Lactate
• Succinate
• Formate, which becomes
• H2
• CO2
• Ethanol
• Acetate

what is added to perform the methyl red test

pH indicator methyl red [dye] and ethanol

Products overcome phosphate buffer to lower pH below 4.2 seen by

adding pH indicator methyl red

below pH 4.2

turns red indicating a positive result

at pH 6.2 and above

turns yellow indicating a negative result

between pH 4.2 to 6.2 

will turn shades of orange which also indicate negative result

Voges-Proskauer test is designed to identify

organisms that ferment glucose and produce products with neutral pH
• Acetoin
• 2,3-butanediol

Procedural Diagram for the Methyl Red and Voges-Proskauer Tests

Voges-Proskauer positive organisms can

produce many of products that methyl red positive organisms can, but turn to producing above products in acid environment

what is added to perform the Voges-Proskauer test

VP reagents

VP reagent A: α-naphthol

VP reagent B: Potassium hydroxide (KOH)

oxidizes acetoin to

diacetyl

Diacetyl reacts with guanidine to

produce red color

Red color change indicates

positive VP test

Weak positive test may

change to pink color

No color change or copper color

indicates negative VP test

(application) methyl red Voges-Proskauer broth is used

• Components of IMViC battery of tests (Indole, Methyl red, Voges-Proskauer, and Citrate) used to distinguish between Enterobacteriaceae family members and differentiate them from other Gram-negative rods
• Escherichia, Shigella, and Salmonella species are methyl red positive whereas Enterobacter, Serratia, and Erwinia species are Voges-Proskauer positive

Methyl Red Test Results and Interpretations

Voges-Proskauer Test Results and Interpretation

Voges-Proskauer Test – Results #1

control: no color change (-). Organism does not perform a 2,3-butanediol fermentation (acetoin was not produced or not detectable)

E. coli: no color change (-). Organism does not perform a 2,3-butanediol fermentation (acetoin was not produced or not detectable).

E. aerogenes: red (+). Organism performs a 2,3-butanediol fermentation (acetoin was produced)

Voges-Proskauer Test – Results #2

control: no color change (-). Organism does not perform a 2,3-butanediol fermentation (acetoin was not produced or not detectable).

E. coli: no color change (-). Organism does not perform a 2,3-butanediol fermentation (acetoin was not produced or not detectable).

E. aerogenes: red (+). Organism performs a 2,3-butanediol fermentation (acetoin was produced)

Methyl Red Test – Results #1

control: no color change (-). Organism does not perform a mixed acid fermentation (acidic end products were not made in detectable amounts).

E. coli: red (+). Organism performs a mixed acid fermentation.

E. aerogenes: no color change (-). Organism does not perform a mixed acid fermentation (acidic end products were not made in detectable amounts).

Methyl Red Test – Results #2

control: no color change (-). Organism does not perform a mixed acid fermentation (acidic end products were not made in detectable amounts).

E. coli: red (+). Organism performs a mixed acid fermentation.

E. aerogenes: no color change (-). Organism does not perform a mixed acid fermentation (acidic end products were not made in detectable amounts).