Question

Why is the ornithine decarboxylation reaction indicated by a purple color and a negative is yellow?

Answer 1

The incubation broth contains the indicator bromocresol purple, which is yellow below pH `5.2` and purple above pH `6.8`.

Explanation:

The broth also contains glucose and ornithine.

Initially,the organism metabolizes the glucose.

This causes the pH to drop, and the colour changes to yellow.

Acidification activates the enzyme ornithine decarboxylase, which catalyzes the decarboxylation of ornithine to putrescine.

If the enzyme is present, the ornithine (pKa `~~ 1.94` at `25^@ "C"`) is decarboxylated and the basic diamine (pKa `~~ 10.8` at `20^@ "C"`) turns the colour back to purple (a positive test).

If the enzyme is absent, the colour stays yellow (a negative test).

In the image below, Tube B shows the colour at the beginning of the test, Tube A shows a positive result, and Tube C shows a negative result.