Consider the decarboxylase base broth for lysine decarboxyla

Consider the decarboxylase base broth (for lysine decarboxylation).

a) Why isn\'t a different base broth required for each decarboxylase medium?

b) Is the base broth a positive or negative control?

c) What information is provided by the base broth?

Solution

Answer

a)

Moller discovered the decarboxylase broth to differentiate the gram-negative enteric bacilli on the basis of these bacteria ability to decarboxylate the amino acids. The decarboxylase broth will detect the presence of lysine and ornithine decarboxylase and arginine dihydrolase.

The medium contains beef extract, peptic digest which acts a nitrogenous source, dextrose which is the fermentable carbohydrates and pyridoxal which acts as a cofactor for decarboxylase enzyme. The decarboxylase is produced only under acidic conditions. A different base broth is not required for each decarboxylase medium because when the medium is inoculated with the dextrose fermenting bacteria, the bacteria will produce acid that lowers the pH of the medium. This change in pH will change the color of the medium from purple to yellow. This change in color will help in predicting whether decarboxylation occurred or not.

If the bacteria produce the enzyme, the amino acids in the medium will degrade and then the pH of the medium will increase resulting in a color change of the medium from yellow to purple resulting in the production of amines. If the bacteria do not produce the suitable enzyme, the pH remains acidic and yellow in color.

b)

The base broth would be a negative control because the bacteria fail to produce the enzyme but utilizes the available carbohydrate source and result in yellow color of the medium. Non-fermenting bacteria cannot utilize the dextrose and remain in the same color as the bacteria in uninoculated control tube of basal medium.  

c)

Only the bacteria that are capable of fermenting the dextrose, decarboxylation occurs and the enzyme is produced resulting in a color change to purple.