Decarboxylation test (lysine catabolism)

What is being tested in decarboxylase media?

To differentiate Enterobacteriaceae

What are the frequently used amino acids in decarboxylation test?

• ornithine

• lysine

• arginine

Can one decarboxylase enzyme break down multiple amino acids?

No, it is substrate-specific

What are products of the decarboxylation of amino acids?

amine and CO₂

Why does decarboxylation raise pH of media?

The accumulation of amines is alkaline, which is basic

Which group of lysine amino acids is removed by decarboxylase?

carboxylic acid

Components of lysine decarboxylase test media

• Peptone

• Glucose

• Bromocresol Purple

What is the purpose of peptone in the media

Contain lysine

What is the purpose of glucose in the media?

glucose is required for fermentation reaction

making the media acidic → turn yellow

What is the indicator of the media?

Bromocresol Purple

What is the color of bromocresol purple if the media is acidic?

yellow

What is the color of bromocresol purple if the media is basic?

purple

What is the purpose of adding mineral oil at the end?

To seal out oxygen, promoting fermentation

What are the requirements for decarboxylating reversion to turn the bromocresol back to purple?

• Low pH which changes the color to yellow

• Presence of lysine

If no color change occurs in the media (clear brown-ish color) , what does this mean?

No glucose fermentation

No lysine decarboxylase

If the media only change to yellow and stops, what does this mean?

Fermentation of glucose, but no lysine decarboxylation 

If the media changes to yellow then changes back to purple, what does this mean?

Fermentation of glucose, lysine decarboxylation

Brief protocol of lysine decarboxylation

1. Obtain 2 lysine broth tubes, label each with species names (E.coli & p.stuartii )

2. Aseptically transfer broth-to-broth each specimen to the labeled tube

3. Add 2-3 cm of mineral oil on top (around 8-10 drops)

4. Incubate for 24 hours at 37°