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Microbiology
Hektoen Agar is a selective medium that helps in detecting and isolating only the Salmonella and Schigella organisms from other Enteric bacteria. Since it can also isolate the growth of salmonella from Schigella sps., it can be classified under differentiating media. The bile salts present in the composition of the medium inhibit the growth of gram-positive organisms in the culture and allow only gram negative bacteria to grow. Sucrose, Salicin and lactose are added into Hektoen Agar in order to allow the growth of pathogens that are lactose fermentors. Ammonium ferric citrate acts as indicator (reacts with substrate to give rise to a color for visualization) and Sodium thio-sulfate reacts with hydrogen sulfide (released by salmonella Sps. but not by Schigella Sps.) and helps in formation of black precipitate with iron (II) present in ferric citrate. Thus black-centered salmonella colonies represent precipitated black ferrous ammonium citrate while Schigella appear as bluish green colonies. The indicators consisting of acid fuchsin and bromothymol blue help in improved recovery of gram negative bacteria. So, the composition of Hektoen Agar is peptone or yeast extract, bile salts, sucrose, lactose, salicin, ferric citrate, sodium chloride, sodium thio-sulfate, acid fuchsin, agar and bromothymol blue