Why isn't a different base broth required for each decarboxylase test?

Moller discovered the decarboxylase broth to distinguish the gram-negative enteric bacilli based on the tendency of these bacteria to decarboxylate the amino acids. The decarboxylase broth will determine the existence of ornithine decarboxylase, lysine, and arginine dihydrolase.  

The medium comprises beef extract, peptic digest that functions as a source of nitrogen, dextrose that is the fermentable carbohydrates, and pyridoxal that functions as a cofactor for decarboxylase enzyme. The decarboxylase enzyme is generated only under acidic conditions.  

A different base broth is not needed for each decarboxylase medium as when the medium is inoculated with the dextrose fermenting bacteria, the bacteria will generate acid, which reduces the pH of the medium. This variation in pH will modify the color of the medium from purple to yellow. This change in color will assist in determining that whether decarboxylation took place or not.  

nacl is added to the aqueous of what? ? in order to get a proper answer you have to specify the exact composition of the aqueous solution.

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