Hektoen Enteric agar

Hektoen Enteric (HE) Agar :

  • Is a selective and differential agar.
  •  primarily used to recover Salmonellaa and Shigella from patient specimens although many other species may grow on these plates.

 

  • The plates contain various sugar sources (lactose, sucrose and salicin), none of which can be used by either Shigella or Salmonella, however the media also includes peptone which can be used as a carbon source.
  • Since most bacteria can utilise the sugars in preference to peptone, these “uninteresting” bacteria acidify the medium and turn a pH indicator yellow or red.
  • Peptone metabolism by Shigella and Salmonella alkalises the medium, turning a pH indicator blue.
  • The presence of thiosulfate or ferricammounium citratein the medium produces a black precipitate in the presence of H2S, allowing Shigella – which does not produce H2S, and appears as green colonies – to be distinguished from Salmonella – which does produce hydrogen sulfide and appears as black colonies.
  • There are very few sulfur-reducing bacteria other than Salmonella which can be isolated from the intestines. Most of these are inhibited on HEK plates by the inclusion of bile salts , so encountering a black colony that is not Salmonella is unusual, although not unheard of. Those that are may be identified as red or yellow colonies with a black centre, indicating that they are fermenting sugar and probably not Salmonella. However, it should be noted that there are rare strains of Salmonella which are capable of lactose fermentation, which will appear in the same way.

 

Hektoen Enteric (HE) Agar

Hektoen Enteric (HE) Agar

Shigella flexneri & Salmonella typhimurium on Hektoen agar

Shigella flexneri & Salmonella typhimurium on Hektoen agar

 

Reference :
wikipedia

 

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