Acetate Differential Agar

Acetate Differential Agar was formulated by Trabulsi and Ewing. Tatum, Ewing and Weaver modified the medium by replacing sodium citrate by sodium acetate, which enables the differentiation of Shigella species from Escherichia coli. Organic acids have been used widely as an aid in the differentiation of Enterobacteriaceae, usually in formulae that contained organic nitrogen sources. Most bacteria, however, can use citrate and acetate in the presence of organic nitrogen.
The differentiation of groups is based on the ability or failure of the test culture to utilize acetate in a medium devoid of trace organic nitrogen. This medium contains sodium acetate as the sole source of carbon. Trabulsi and Ewing demonstrated that Shigella and Proteus species are unable to utilize acetate and therefore fails to grow. Majority of Escherichia coli and closely related organisms grow well within 24-48 hours but some strains grow very slowly and a few strains are unable to utilize acetate as a sole carbon source. Acetate utilization is indicated by formation of blue colour, which is due to the utilization of sodium acetate and subsequent formation of an alkaline reaction detected by the presence of bromothymol blue indicator.

for the differentiation of Shigella species from E.coli and non fermantative gram-negative bacilli