The thiosulphate and ferric citrate are present to detect H 2 S-producing organisms. Hoben et al. Technique Inoculate the medium with fresh faeces suspended in Ringers solution or inoculate directly with rectal swabs. Spread the inoculum to obtain well separated colonies. Further incubation will improve differentiation between shigellae and salmonellae.
Appearance Dehydrated medium: Pale green coloured, free-flowing powder Prepared medium: Green to dark green coloured gel. Precautions Do not overheat the medium or hold it molten for long periods. Proteus species may resemble salmonellae or shigellae. Further testing must be carried out to confirm the presumptive identification of organisms isolated on this medium.
References 1. King S. Since the enteric pathogens, Salmonella and Shigella can tolerate these inhibitory substances they generally grow faster and larger than the coliforms. The production of H2S by certain enteric gram-negative rods, such as Salmonella , can be detected on HE agar due to the addition of thiosulfate and ferric ammonium citrate to the formula.
Salmonella produces bacterial enzymes that cause a sulfide molecule to be released from the thiosulfate present in the media. This sulfide molecule then couples with a hydrogen ion to form H2S gas.
The H2S gas reacts with the ferric ammonium citrate, forming a precipitate, resulting in colonies that are black or have a black center. Other nonpathogenic enteric organisms, such as Proteus spp. If these organisms can overcome the inhibitory effects of the bile salts and grow, they usually can be differentiated from the pathogens because Proteus and Citrobacter freundii can utilize at least one of the carbohydrates present in the HE agar.
An orange-yellow colony with a black center is most likely not an intestinal pathogen, although rare strains of Salmonella are capable of lactose fermentation and would appear this way. The fermentation of carbohydrates lactose, sucrose, and salicin , is one of the differentiating characteristics used to identify the coliforms. Salmonella and Shigella are unable to utilize these three specific carbohydrates, whereas most nonpathogenic coliforms can use at least one of them.
Acids may be produced by carbohydrates fermentation, and acid fuchsin reacting with thymol blue produces the yellow color when the pH is lowered. Since HE agar is primarily a screening agar, additional testing is required to confirm or rule out Salmonella or Shigella. Several options are available for confirmatory testing ranging from commercial identification kits to tubed biochemical e. This site uses Akismet to reduce spam. Learn how your comment data is processed.
News Ticker. Stool culture on Hektoen enteric agar: mixed flora including Escherichia coli red arrow , Salmonella blue arrow , and Proteus vulgaris yellow arrow. Yellow-orange colonies of Proteus vulgaris in Hektoen Enteric Agar. About Acharya Tankeshwar Articles.
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