Question

"if a bacterium cannot use citrate, what result will occur in citrate agar?"

Answer 1

The answer is it will not change it's color.

A citrate is a derivative of citric acid; that is, the salts , esters , and the polyatomic anion found in solution. An example of the former, a salt is trisodium citrate ; an ester is triethyl citrate .

Answer 2

If a bacterium cannot use citrate agar will not change its color (stays green). On the other hand, if bacteria have the ability to use citrate, the medium will change its color from green to blue.

This happens because citrate agar contains pH indicator such as bromothymol blue which transforms from green to blue in alkaline conditions.