Acetate Differential Agar is recommended for the differentiation of Shigella species from Escherichia coli based on utilization of acetate. Acetate utilization is indicated by formation of blue color, due to the utilization of sodium acetate and subsequent formation of an alkaline reaction detected by the presence of bromothymol blue indicator. Citrobacter freundii, Enterobacter cloacae, Escherichia coli, Klebsiella pneumoniae, Salmonella Arizonae gives positive reaction. Salmonella Typhi and Shigella sonnei do not utilize acetate hence show negative reaction. Color of the medium remains green.
- Appearance: Cream to light green homogeneous free flowing powder
- Gelling: Firm, comparable with 2.0% agar gel
- Color and Clarity of prepared medium: Emerald green colored clear to slightly opalescent gel forms in tubes as slants
- Reaction of 2.92% w/v aqueous solution at 25°C. pH : 6.7±0.2
- Cultural characteristics observed after an incubation at 25-30°C for upto 1-7 days