Hektoen Enteric Agar Medium

Principle And Interpretation

Hektoen Enteric Agar, a selective and differential medium designed to isolate and differentiate members of the species Salmonella and Shigella from other Enterobacteriaceae and was developed by King and Metzger (1,2). When compared with other selective medium, this medium inhibits the growth of Salmonella and Shigella very slightly; thus giving high yields of these microorganisms, but at the same time inhibits accompanying gram positive and other microorganisms. This medium is recommended by United States Pharmacopoeia, 2019 for testing the presence of Salmonella in dietary supplements (3). This medium is recommended in testing of Salmonella in food sample by various standards (4,5,6). Compared to other differentiating media commonly used in clinical laboratories, Hektoen Enteric Agar is efficient in increasing the isolation rate of Salmonella sp. Bile salts, bromthymol blue and acid fuchsin inhibit the growth of most Gram positive organisms. Lactose, salicin and sucrose, serves as fermentable source of carbohydrates to encourage the growth and differentiation of enteric bacteria. In this medium by increasing the carbohydrate and peptone content of the medium the inhibitory effect of bile salts and indicators are countered. Proteose peptone provides nitrogen, carbon, and amino acids required for organism growth. Yeast Extract is a vitamin source. Sodium chloride maintains the osmotic balance of the medium. Sodium thiosulfate provides a source of sulfur. Hektoen Enteric Agar can also detect the production of hydrogen sulfide gas, which turns parts of the medium black. Ferric ammonium citrate serves as iron source, which cause production of hydrogen sulfide from sodium thiosulphate and also aids in the visualization of hydrogen sulfide production by reacting with hydrogen sulfide gas to form a black precipitate. Enterobacters that are capable of fermenting one or more of the carbohydrates produces yellow or salmon-orange coloured colonies like Klebsiella pneumoniae, that ferments lactose. Non-fermenters will produce blue-green colonies. Organisms that reduce sulfur to hydrogen sulfide will produce black colonies or blue-green colonies with a black center. Salmonella reduce sulfur to hydrogen sulfide, producing a black precipitate. Micrococcus luteus does not grow.