Hektoen Enteric Agar is used for differential & selective isolation of Salmonella and Shigella from samples.
Hektoen Enteric Agar is formulated by King and Metzger to isolate Shigella and Salmonella species. The medium contains Bile salt which is inhibitory to Gram-positive bacteria and many Gram- negative bacteria. The medium contains Proteose peptone as an essential amino acid and nitrogen supplement. Yeast extract is added as the source of vitamins and other growth factors. Sodium chloride helps maintaining the osmotic balance of the cells in the medium. Lactose present in the medium makes helps to differentiate lactose fermenters from non- lactose fermenters like Salmonella and late-lactose fermenters like Shigella. Hektoen Enteric Agar contains other carbon sources like Sucrose and Salicin for optimal differentiation of enteric pathogens based on their ability to ferment different sugars. Acid fuschin and Bromo thymol blue acts as a pH indicator in the medium. E. coli is partially inhibited wherein they grow to a limited extent utilizing all the three sugars like lactose, Sucrose and Salicin; forming salmon or orange coloured colonies in the medium. Sodium thiosulphate and Ferric ammonium citrate helps in differentiation of H2S producing coliforms. The H2S producing coliforms like Proteus and Salmonella grow in the medium forming clear colony with a black center. Agar is added in the medium as a solidifying agent.