Biology 250. Carbohydrate Catabolism

Vocabulary: substrate, enzyme, product, pH indicator

Starch hydrolysis

The enzyme amylase converts its substrate, starch, to its products, simple sugars.

Starch left in the agar plus iodine turns black.

Whereever the starch has been converted to sugar, the agar is clear after adding iodine.

Carbohydrate fermentation in phenol red broths

Various enzymes convert the substrate sugars into the products, acids and/or gasses.

In the presence of acid, phenol red changes color from red to yellow, a positive reaction.

In the presence of gases, the space inside the Durham tube upside down in the broth fills with the gas it traps. If it is more than 10% full of gas, it is a positive reaction.

Carbohydrate fermentation in the MR-VP (methyl red-Voges Proskauer) tests

The MR-VP tests determine what products an organism makes when it breaks down glucose. They are not usually both positive.

Methyl red positive: If an organism converts all the glucose available into acidic products, it will produce a large amount of acid, which will be enough to change the color of the methyl red pH indicator from yellow to red (positive reaction).

Methyl red negative: If an organism converts some of the glucose available to neutral products, there will not be enough acid produced to change the color of the methyl red, and the test will be negative/yellow.

Voges-Proskauer positive: If an organism converts some of the glucose available to neutral products, these can be detected by the reagents used in the VP test. The neutral product acetoin plus Barritt's A (VP-A, alpha-naphthol) plus Barritt's B (VP-B, KOH) plus lots of oxygen and warming during a 10 minute minimum incubation period will produce a ruby red color, a positive reaction.

Voges-Proskauer negative: If the organism doesn't produce the neutral product, adding the reagents will produce a brownish-green color, a negative reaction.