Hektoen Enteric Agar

General Information About Hektoen Enteric Agar

Hektoen Enteric Agar (HE) is a selective and differential medium designed to isolate and differentiate members of the species Salmonella and Shigella from other Enterobacteriaceae.  

The Basic Chemistry of HEktoen enteric agar

Bile salts and the dyes bromthymol blue and acid fuchsin inhibit the growth of most Gram positive organisms.  

Lactose, sucrose, and salicin provide fermentable carbohydrates to encourage the growth and differentiation of enterics. 

Ferric ammonium citrate provides a source of iron to allow production of hydrogen sulfide from sodium thiosulfate, which provides a source of sulfur. Ferric ammonium citrate also allows the visualization of hydrogen sulfide production by reacting with hydrogen sulfide gas to form a black precipitate. 

Reading the Results

Enterics that ferment one or more of the carbohydrates will produce yellow to salmon-colored colonies. 

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.