Hektoen Enteric Agar

This medium was initially intended for the selective isolation of Shigella spp. but can also be used for Salmonella spp. primarily from clinical specimens although it has been used in the examination of dairy products. The medium contains high levels of Peptone to counteract some of the toxic effects, particularly on Shigella, of the Bile Salts used to make the medium selective. In addition to Lactose, Sucrose and Salicin are also included allowing for improved differentiation than with Lactose only. The presence of H2S is detected by the reaction between Ammonium Ferric Citrate and Sodium Thiosulphate. A double indicator system of Acid Fuchsin and Bromothymol Blue is helpful in the differentiation process.