Question

1- On occasion, you will see no color change in the medium, however, there is visible growth on the Simmons citrate agar surface. How would you interpret this result?

2- In most biochemical test media, you will find a source of protein such as casein, beef extract or peptone. Why does Simmons citrate agar not have any such materials?

Answer 1

1) On occasion, you will see no color change in the medium but there is visible growth on the Simmons citrate agar surface. This is due to citrate negative organisms.

Citrate negative organisms grow weakly on the Simmon citrate agar surface and cannot utilize citrate as their carbon source. So do not show any change in the color and remain green in color.

For example : Escherichia coli, Salmonella typhi

2) In most biochemical test media, protein sources are casein, beef extract, peptone whereas in Simmons citrate agar ammonium salt is the source.

The organisms that grow on Simmons citrate agar plate can metabolize ammonium salt in the medium which acts as sole source of nitrogen.

The metabolism of ammonium salt produce ammonia that increases the pH of the medium which causes the color change in the bromothymol blue indicator.

Protein sources like casein, beef extract, peptone cannot produce alkali substances like ammonia which increases the pH and cannot show change in the color of the indicator. Hence Simmons citrate agar not have any such materials.