Hektoen enteric agar was developed by King and Metzger as a differential selective medium for the isolation of Shigella spp. and Salmonella spp. species from enteric pathological samples.

The meat peptone and yeast extract provide the required nitrogen, carbon and vitamins. Lactose, sucrose and salicin are fermentable carbohydrates. Bromothymol blue is added as a pH indicator in order to identify carbohydrate fermenting organisms. The combination of ferric ammonium citrate and sodium thiosulfate allows the production of hydrogen sulphide. Hydrogen sulphide positive colonies produce black centred colonies. Sodium chloride maintains the osmotic balance. The bile salts and acid fuchsin inhibit Gram-positive organisms.

Preparation

Suspend 76g of the medium in one litre of deionised / purified water. Allow the medium to soak whilst mixing for 10 minutes. Heat with frequent agitation and boil for one minute to completely dissolve the medium. Do not autoclave this medium. Cool to 45-50°C and aseptically dispense into appropriate sterile containers