Hektoen Enteric Agar is a differential agar medium used for the isolation and identification of pathogenic Enterobacteriaceae from clinical specimens. The medium was developed for the isolation of Shigella sp. However, Salmonella sp. may also be isolated and identified using this medium. Bile salts are used in this medium to inhibit the growth of Gram positive bacteria. The inhibitory action of this medium has been tempered by increasing the concentration of peptones and carbohydrates and thus allowing better recovery and growth of Salmonella and Shigella sp. Bromthymol blue and acid fuchsin are used as acid-base indicators in this medium. Enteric bacteria which utilize lactose, sucrose or salicin with acid production give yellow-orange to salmon colored colonies. Salmonella and Shigella colonies are green to bluish green in color. Hydrogen sulfide producing bacteria, such as Salmonella, will produce black centered colonies in the presence of ferric ammonium citrate and sodium thiosulfate.

Beige with green tint, homogeneous, free flowing powder

Light to dark green, clear to trace hazy

7.6 ± 0.2

Enterobacter aerogenes ATCC 13048 Growth/yellow to salmon colored colonies with bile precipitate Escherichia coli ATCC 25922 Inhibited Proteus vulgaris ATCC 13315 Inhibited Salmonella typhimurium ATCC 14028 Growth/green w/black centers Shigella flexneri ATCC 12022 Growth/blue-green colonies

Mix 75g per liter of ddH2O until evenly dispersed. Heat with repeated stirring and boil ntil completely solubilized. Do not autoclave. Pour plates can be prepared after the medium has cooled to 45-50.0°C.

Store powdered media at 4°C. Opened bottles should be capped tightly and kept in a dark, low humidity environment. Prepared media should be kept at 4°C and used within a short period of time.