Hektoen Enteric agar - Epidemiology

What is Hektoen Enteric Agar?

Hektoen Enteric (HE) agar is a selective and differential culture medium primarily used for isolating and differentiating enteric pathogens, particularly members of the genera Salmonella and Shigella. It was developed by King and Metzger in 1968 and is commonly employed in clinical microbiology laboratories and public health settings.

Composition and Function

HE agar contains several key ingredients that enhance its selectivity and differential capabilities. These include:

Bile salts - Inhibit the growth of Gram-positive bacteria and most non-pathogenic Gram-negative bacteria.
Lactose, sucrose, and salicin - Serve as fermentable carbohydrates.
Bromthymol blue and acid fuchsin - pH indicators that facilitate differentiation based on fermentation.
Ferric ammonium citrate and sodium thiosulfate - Detect hydrogen sulfide (H₂S) production.

How Does It Work?

HE agar differentiates bacteria based on their ability to ferment carbohydrates and produce hydrogen sulfide. Pathogenic bacteria like Salmonella and Shigella do not ferment lactose, sucrose, or salicin, leading to the formation of green or blue-green colonies. Non-pathogenic enteric bacteria that ferment these carbohydrates produce yellow to salmon-pink colonies. Additionally, Salmonella spp. produce hydrogen sulfide, which reacts with ferric ammonium citrate to form black precipitates, distinguishing them from Shigella spp.

Importance in Epidemiology

HE agar plays a critical role in epidemiological investigations of foodborne outbreaks, waterborne diseases, and other public health concerns. Its ability to rapidly isolate and differentiate pathogenic enteric bacteria makes it invaluable in:

Outbreak detection and investigation - Identifying the source of an outbreak and implementing control measures.
Surveillance - Monitoring the prevalence and spread of enteric pathogens in the population.
Antimicrobial resistance tracking - Studying resistance patterns in isolated pathogens.

Advantages and Limitations

HE agar offers several advantages, including:

High selectivity for pathogenic enteric bacteria.
Clear differentiation between Salmonella and Shigella.
Rapid results facilitating timely epidemiological interventions.

However, it also has limitations:

Some strains of non-pathogenic bacteria may occasionally grow on HE agar, potentially causing confusion.
Requires confirmatory tests to definitively identify isolated pathogens.

Confirmatory Tests

Isolates from HE agar are typically subjected to further biochemical and serological tests to confirm their identity. Common tests include:

Triple Sugar Iron (TSI) agar - Differentiates bacteria based on carbohydrate fermentation and H₂S production.
Urease test - Determines the ability to hydrolyze urea.
Serotyping - Identifies specific serovars of Salmonella and Shigella.

Conclusion

In summary, Hektoen Enteric agar is a vital tool in the field of epidemiology for isolating and differentiating enteric pathogens. Its selective and differential properties enable efficient identification of pathogens like Salmonella and Shigella, aiding in outbreak investigations, surveillance, and antimicrobial resistance tracking. Despite its limitations, HE agar remains a cornerstone in public health microbiology, providing essential data for safeguarding public health.