Explain how an organism that possesses the citrate lyase enzyme

  Simmon’s Citrate agar is used to determine an organism’s ability to use citrate as a sole carbon source.Simon’s citrate agar is a defined medium in which sodium citrate is the sole carbon source, and ammonium is the sole nitrogen source. Bromothymol blue (BTB) is included as a pH indicator 
The medium is initially at pH 6.9, at which BTB is green; at a pH greater than 7.6 BTB turns a deep blue. The pH change is induced by CO2, which is given off as a by-product of citrate utilization. When it reacts with Na and H2O in the agar it raises the pH above 7.6 

The organism must contain the enzyme citrase to degrade citrate 

(citrase) 
Citrate >CO2 + Na HCO + H2O 
I
(blue color change) 

Now, coming to how a citrate test may give false results. There are only 2 ways that can happen. 
- Failure to incubate with loose caps may result in a false-negative results. 
- Avoid using a large inoculum to streak the strand; an inoculum that is too heavy may result in a false-positive test. 

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