Question

Is HE agar a defined or an undefined medium? Provide the reasoning behind your choice and explain why this formulation is desirable.

Answer 1

Hektoen Enteric (HE) agar is a complex, undefined, selective and differential medium useful for isolating and differentiating Gram-negative enteric pathogens. This medium is particularly useful in the isolation of Shigella species. HE agar is useful as a plating media for the culture of Enterobacteriaceae from stool specimens due to its moderately selective nature as well as for its differentiation property.
HA agar is used as a direct and an indirect plating medium to recover gastrointestinal pathogens, such as Salmonella and Shigella, from food, water, and fecal samples suspected of containing these organisms. As a selective medium, it inhibits most non-pathogenic (normal flora) enteric organisms and assists in selective recovery of Salmonella and Shigella from feces and as a differential medium it allows microbiologists to note visual differences in colony morphology and quickly eliminate nonpathogenic gram-negative rods from pathogenic gram-negative rods with minimal additional testing. For example,
(a) Fermentation of lactose, sucrose or salicin results in the production of acid which give rise to yellow-orange to salmon colored colonies.
(b) Colonies of Salmonella and Shigella spp. are green to bluish-green in color.
(c) Salmonella sp. that produce H2S appear as blue-green colonies with black centers. H2S producers form black-centered colonies in the presence of ferric ammonium citrate and sodium thiosulfate.