Why is the citrate test carried out on an agar slope rather than a broth?

Answer 1

Background:

Simmons' Citrate Agar is a defined, selective medium that tests for an organism's ability to use citrate as a sole carbon source and ammonium ions as the sole nitrogen source.

The medium contains citrate, ammonium ions, and other inorganic ions needed for growth.

It also contains bromothymol blue, a pH indicator. Bromothymol blue is green at pH below 6.9, and then turns blue at a pH of 7.6 or greater.

SO:

Sometimes, it is possible to detect growth on the citrate slant without the accompanying color change to blue. This is most likely due to insufficient incubation. Either a combination of blue color and growth or growth alone without the blue color should be scored as a positive for the citrate use test.

Answer 2

The citrate test is carried out on an agar slope rather than a broth because it allows for better observation of growth along the slant, which can indicate the utilization of citrate as a carbon source by the organism being tested. In a broth, it may be harder to detect growth in response to citrate utilization due to the homogeneity of the medium.