Question

The lysine decarboxylase test medium contained 5 grams of lysine and 1 gram of glucose per liter.  
Why is glucose important for this test to work? Also, how would reversing the amounts of lysine and glucose in the test medium (i.e., using 1 gram of lysine and 5 grams of glucose instead) affect the outcome of the test and why?

Answer 1

Lysine decarboxylase test is useful to know whether microorganisms can use lysine to use as carbon source during their growth. During the test process microbes are added to the lysine decarboxylase medium. In the medium there is also present a pH indicator which will increased when the lysine present in medium is used up. This can be visualized by colour change. At the beggining of the experiments, microbes use the glucose first. Once glucose is used up, this will reduce the pH of the medium. This will be verified by a colour change purple to yellow. In acidic medium Lysine decarboxylase gets activated. Next, the microbes are incubated further so that they will use the lysine in the medium. Once lysine is used up the colour of the medium once again will be back to purple again.

Glucose is added to the medium for the fermentation of the microorganisms and is required for the ongoing of the test process.

If we add less lysine and more glucose, this will change the outcome of the test. As larger amount of glucose will make the pH of the medium too acidic which might not be proper for the functioning of lysine decarboxylase. Secondly, lower amount of lysine might not sufficient for the microbes to use for.