Hektoen Enteric Agar (HEA) – Technical Specifications
Selective & Differential Medium for Enteric Pathogens
1. Intended Use
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Primary Application: Isolation/differentiation of Salmonella and Shigella spp.
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Sample Types: Clinical stool specimens, food, environmental samples
2. Key Functional Components
| Component | Role |
|---|---|
| Bile salts | Inhibits Gram-positive & non-pathogenic Gram-negative bacteria |
| Lactose/sucrose/salicin | Carbohydrate sources for fermentation |
| Sodium thiosulfate + Ferric ammonium citrate | H₂S production detection |
| Bromothymol blue + Acid fuchsin | pH indicators (differentiate fermenters) |
3. Physical Characteristics
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Color: Green (prepared medium)
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Clarity: Clear to slightly opalescent
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pH: 7.5 ± 0.2 at 25°C
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Format: Dehydrated powder
Performance Characteristics
Colony Morphology
| Organism | Colony Appearance | Biochemical Basis |
|---|---|---|
| E. coli (coliform) | Salmon-pink, may yellow medium | Rapid lactose fermentation |
| Salmonella spp. | Blue-green with black centers | H₂S production (thiosulfate → FeS) |
| Shigella spp. | Pale green, translucent | Non-lactose fermenter |
| Proteus spp. | Colorless with blackening | H₂S production only |
Quality Control Parameters
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Sterility: No growth after 24h at 35°C
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Growth Promotion:
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Salmonella enterica ATCC 14028: Blue-green colonies with H₂S
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Shigella flexneri ATCC 12022: Pale green colonies
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Selectivity: ≥80% inhibition of E. coli ATCC 25922
Protocol Summary
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Preparation: Suspend 75g/L, boil to dissolve, autoclave (121°C, 15 min)
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Plating: Pour into Petri dishes (~20mL/plate)
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Inoculation: Streak for isolation
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Incubation: 35-37°C for 18-24h (extend to 48h for low-density samples)
Regulatory & Storage
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Shelf Life: 3 years (unopened, <30°C)
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Safety: Handle as potential biohazard post-inoculation
Advantages Over Competing Media
✔ Dual differentiation (fermentation + H₂S)
✔ Superior Shigella recovery vs. SS Agar
✔ Color contrast enhances readability
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