Hektoen enteric agar

A selective and differential culture medium for isolating enteric pathogens

Hektoen enteric agar (HEA) is a type of microbiological culture medium that is both selective and differential. It is primarily used for the isolation and differentiation of enteric bacteria, particularly those belonging to the genera Salmonella and Shigella, from clinical specimens and food samples.

Composition

Hektoen enteric agar contains several key ingredients that make it effective for its intended purpose:

• Peptone: Provides essential nutrients for bacterial growth.
• Bile salts: Inhibit the growth of Gram-positive bacteria, allowing for the selective growth of Gram-negative bacteria.
• Lactose, sucrose, and salicin: Serve as fermentable carbohydrates.
• Bromothymol blue and acid fuchsin: pH indicators that help differentiate between lactose fermenters and non-fermenters.
• Ferric ammonium citrate and sodium thiosulfate: Allow for the detection of hydrogen sulfide production, which is characteristic of some Salmonella species.

Mechanism of Action

Hektoen enteric agar works by exploiting the metabolic differences between enteric bacteria. The medium's selective agents, such as bile salts, suppress the growth of non-target organisms, while the differential agents allow for the visual distinction between different types of bacteria based on their metabolic properties.

• Lactose fermenters: Bacteria that can ferment lactose, sucrose, or salicin produce acid, leading to a color change in the medium. Colonies appear yellow to salmon-pink.
• Non-lactose fermenters: Bacteria that do not ferment these carbohydrates remain colorless or take on the color of the medium.
• Hydrogen sulfide producers: Bacteria that produce hydrogen sulfide will form colonies with black centers due to the reaction with ferric ammonium citrate.

Applications

Hektoen enteric agar is widely used in clinical laboratories for the isolation of Salmonella and Shigella from stool samples. It is also employed in food microbiology to test for the presence of these pathogens in food products.

Gram-negative Shigella sonnei bacteria cultured on Hektoen enteric agar

Limitations

While HEA is effective for isolating Salmonella and Shigella, it is not entirely specific. Other enteric bacteria, such as Proteus and Citrobacter, may also grow on this medium and produce similar colony morphologies. Therefore, further biochemical testing is often required to confirm the identity of the isolates.

Related pages

• Microbiological culture
• Selective medium
• Differential medium
• Salmonella
• Shigella